HOME

TheInfoList




A computer network is a set of
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

computer
s sharing resources located on or provided by
network nodesIn telecommunications networks, a node (Latin ''nodus'', ‘knot’) is either a redistribution point or a communication endpoint. The definition of a node depends on the network and protocol layer referred to. A physical network node is an electro ...

network nodes
. The computers use common
communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networ ...
s over
digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics comprises the physics, engineering, technology a ...
interconnections to communicate with each other. These interconnections are made up of
telecommunication network A telecommunications network is a group of nodes In general, a node is a localized swelling (a "knot") or a point of intersection (a Vertex (graph theory), vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathema ...
technologies, based on physically wired, optical, and wireless radio-frequency methods that may be arranged in a variety of
network topologies Network topology is the arrangement of the elements (links Link or Links may refer to: Places * Link, West Virginia, an unincorporated community in the US * Link River, Klamath Falls, Oregon, US People with the name * Link (singer) (Linco ...
. The nodes of a computer network may include
personal computer A personal computer (PC) is a multi-purpose computer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or technician ...
s, servers,
networking hardware Networking hardware, also known as network equipment or computer networking devices, are electronic devices which are required for communication and interaction between devices on a computer network A computer network is a group of computer ...
, or other specialised or general-purpose
hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (1 ...
. They are identified by
network address A network address is an identifier for a node In general, a node is a localized swelling (a "knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, ...
es, and may have
hostname In computer networking, a hostname (archaically nodename) is a label that is assigned to a device connected to a computer network A computer network is a group of computers that use a set of common communication protocols over digital signal ...
s. Hostnames serve as memorable labels for the nodes, rarely changed after initial assignment. Network addresses serve for locating and identifying the nodes by communication protocols such as the
Internet Protocol The Internet Protocol (IP) is the network layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model A conceptual model is a representation of a system, made of the composition of concept C ...
. Computer networks may be classified by many criteria, including the
transmission medium A transmission medium is a system or substance that can mediate the propagation of signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such ...
used to carry signals,
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thro ...
,
communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networ ...
s to organize network traffic, the network size, the topology,
traffic control Traffic management is a key branch within logistics Logistics is generally the detailed organization and implementation of a complex operation. In a general business sense, logistics is the management of the flow of things between the point of ...
mechanism, and organizational intent. Computer networks support many
applications Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a co ...
and
services Service may refer to: Activities :''(See the Religion section for religious activities)'' * Administrative service, a required part of the workload of Faculty (academic staff), university faculty * Civil service, the body of employees of a governm ...
, such as access to the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system An information system (IS) is a formal, sociotechnical Sociotechnical systems (STS) in organizational development is an approach to complex organizational ...
,
digital video Digital video is an electronic representation of moving visual images (video) in the form of encoded digital data. This is in contrast to analog video, which represents moving visual images in the form of analog signals. Digital video comprises ...
,
digital audio Digital audio is a representation of sound recorded in, or converted into, Digital signal (signal processing), digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical Sampling (signal processing), s ...
, shared use of application and storage servers, printers, and
fax machines
fax machines
, and use of
email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_dev ...

email
and
instant messaging Instant messaging (IM) technology is a type of online chat Online chat may refer to any kind of communication over the Internet that offers a real-time text, real-time transmission of text-based, text messages from sender to receiver. Chat ...

instant messaging
applications.


History

Computer networking may be considered a branch of
computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , and . Computer science ...
,
computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The di ...
, and
telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin in the desire of humans for communication over a distance greater than that feasible with the , but with ...
, since it relies on the theoretical and practical application of the related disciplines. Computer networking was influenced by a wide array of technology developments and historical milestones. * In the late 1950s, a network of computers was built for the U.S. military
Semi-Automatic Ground Environment The Semi-Automatic Ground Environment (SAGE) was a system of large computers and associated networking equipment that coordinated data from many radar Radar (radio detection and ranging) is a detection system that uses radio waves to det ...
(SAGE)
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
system using the
Bell 101 modem The Bell 101 dataset or Bell 101 modem was the first commercial modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with speciali ...
. It was the first commercial
modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telep ...

modem
for computers, released by
AT&T Corporation AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet The Internet (Capitalization of Internet, or internet) is the global system of i ...
in 1958. The modem allowed
digital data Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digit (unit), digits. A ...
to be transmitted over regular unconditioned telephone lines at a speed of 110
bits per second In telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It has its origin in the desire of huma ...
(bit/s). * In 1959,
Christopher Strachey Christopher S. Strachey (; 16 November 1916 – 18 May 1975) was a British computer scientist A computer scientist is a person A person (plural people or persons) is a being that has certain capacities or attributes such as reason, moralit ...
filed a patent application for
time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
and
John McCarthyJohn McCarthy may refer to: Government * John George MacCarthy (1829–1892), Member of Parliament for Mallow constituency, 1874–1880 * John McCarthy (Irish politician) (1862–1893), Member of Parliament for the Mid Tipperary constituency, 189 ...
initiated the first project to implement time-sharing of user programs at MIT.F. J. Corbató, et al.,
The Compatible Time-Sharing System A Programmer's Guide
' (MIT Press, 1963) . "Shortly after the first paper on time-shared computers by C. Strachey at the June 1959 UNESCO Information Processing conference, H. M. Teager and J. McCarthy at MIT delivered an unpublished paper "Time-shared Program Testing" at the August 1959 ACM Meeting."
Stratchey passed the concept on to
J. C. R. Licklider
J. C. R. Licklider
at the inaugural
UNESCO Information Processing Conference
UNESCO Information Processing Conference
in Paris that year. McCarthy was instrumental in the creation of three of the earliest time-sharing systems (
Compatible Time-Sharing System The Compatible Time-Sharing System (CTSS) was one of the first time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algori ...
in 1961,
BBN Time-Sharing System The BBN Time-Sharing System was an early time-sharing system In computing, time-sharing is the sharing of a computing resource among many users at the same time by means of multiprogramming and computer multitasking, multi-tasking.DEC Timesharin ...
in 1962, and
Dartmouth Time Sharing System The Dartmouth Time-Sharing System (DTSS) is a discontinued operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operat ...
in 1963). * In 1959,
Anatoly Kitov Anatoly Ivanovich Kitov (9 August 1920, Samara Samara ( rus, Сама́ра, p=sɐˈmarə), known from 1935 to 1991 as Kuybyshev (; ), is the largest city and administrative centre of Samara Oblast. The city is located at the confluence of the V ...
proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres. * In 1960, the commercial airline reservation system semi-automatic business research environment (SABRE) went online with two connected
mainframes A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic ...
. * In 1963,
J. C. R. Licklider
J. C. R. Licklider
sent a memorandum to office colleagues discussing the concept of the " Intergalactic Computer Network", a computer network intended to allow general communications among computer users. * Throughout the 1960s,
Paul Baran Paul Baran (born Pesach Baran ; April 29, 1926 – March 26, 2011) was a Polish-American engineer who was a pioneer in the development of computer networks. He was one of the two independent inventors of packet switching, which is today the domin ...
and
Donald Davies Donald Watts Davies, (7 June 1924 – 28 May 2000) was a Welsh computer scientist A computer scientist is a person who has acquired the knowledge of computer science Computer science deals with the theoretical foundations of informati ...

Donald Davies
independently developed the concept of
packet switching In telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or re ...
to transfer information between computers over a network. Davies pioneered the implementation of the concept. The
NPL network The NPL network, or NPL Data Communications Network, was a local area computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These i ...
, a local area network at the
National Physical Laboratory (United Kingdom) The National Physical Laboratory (NPL) is the national measurement standards laboratory of the United Kingdom. It is one of the most extensive government laboratories in the UK and has a prestigious reputation for its role in setting and maintai ...
used a line speed of 768 kbit/s and later high-speed T1 links (1.544
Mbit/s In telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin in the desire of humans for communication over a distance greater than that fea ...
line rate). * In 1965,
Western Electric The Western Electric Company was an American electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics ...
introduced the first widely used
telephone switch manually connecting calls with cord pairs at a telephone switchboard A telephone exchange, telephone switch, or central office is a telecommunications Telecommunication is the transmission of information Information can be thought of as ...
that implemented computer control in the switching fabric. * In 1969, the first four nodes of the
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switching network with distributed control and one of the first networks to implement the Internet protocol suite, TCP/IP protocol suite. Both technologies ...
were connected using 50 kbit/s circuits between the
University of California at Los Angeles The University of California, Los Angeles (UCLA) is a public In public relations and communication science, publics are groups of individual people, and the public (a.k.a. the general public) is the totality of such groupings. This is a ...
, the
Stanford Research Institute SRI International (SRI) is an American nonprofit organization, nonprofit scientific research, scientific research institute and organization headquartered in Menlo Park, California. The trustees of Stanford University established SRI in 1946 as ...
, the
University of California at Santa Barbara The University of California, Santa Barbara (UC Santa Barbara or UCSB) is a public land-grant research university A research university is a university A university ( la, universitas, 'a whole') is an educational institution, institution ...
, and the
University of Utah The University of Utah (U of U, UofU, or simply The U) is a public In public relations Public relations (PR) is the practice of deliberately managing the release and spread of information between an individual or an organization (such ...

University of Utah
. In the early 1970s,
Leonard Kleinrock Leonard Kleinrock (born June 13, 1934) is an American computer scientist. A professor at UCLA's Henry Samueli School of Engineering and Applied Science, he made several important contributions to the field of computer science, in particular to ...
carried out mathematical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET. His theoretical work on
hierarchical routingHierarchical routing is a method of routing in networks that is based on hierarchical network address, addressing. Background Most Transmission Control Protocol/Internet Protocol (Internet protocol suite, TCP/IP) routing is based on a two-level hie ...
in the late 1970s with student
Farouk Kamoun Farouk Kamoun (born October 20, 1946) is a Tunisian computer scientist A computer scientist is a person A person (plural people or persons) is a being that has certain capacities or attributes such as reason, morality, consciousness or s ...

Farouk Kamoun
remains critical to the operation of the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
today. * In 1972, commercial services were first deployed on public data networks in Europe, which began using
X.25 X.25 is an ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPE ...
in the late 1970s and spread across the globe. The underlying infrastructure was used for expanding
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A commu ...
networks in the 1980s. * In 1973, the French
CYCLADES The CYCLADES computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by the Node (ne ...

CYCLADES
network was the first to make the
hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (1 ...
responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. * In 1973,
Robert Metcalfe Robert (Bob) Melancton Metcalfe (born April 7, 1946) is an engineer and entrepreneur from the United States who helped pioneer the Internet The Internet (or internet) is the global system of interconnected s that uses the (TCP/IP) t ...
wrote a formal memo at
Xerox PARC PARC (Palo Alto Research Center; formerly Xerox PARC) is a research and development company in Palo Alto, California. Founded in 1969 by Jack Goldman, Jacob E. "Jack" Goldman, Xerox Corporation's chief scientist, the company was originally a divis ...
describing
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
, a networking system that was based on the Aloha network, developed in the 1960s by
Norman Abramson Norman Manuel Abramson (April 1, 1932biography
from
University of Hawaii A university ( la, universitas, 'a whole') is an institution Institutions, according to Samuel P. Huntington, are "stable, valued, recurring patterns of behavior". Institutions can refer to mechanisms which govern the behavior Behavio ...

University of Hawaii
. In July 1976, Robert Metcalfe and
David Boggs David Reeves Boggs (born 1950) is an electrical and radio engineer from the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily lo ...
published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks" and collaborated on several patents received in 1977 and 1978. * In 1974,
Vint Cerf Vinton Gray Cerf (; born June 23, 1943) is an American Internet pioneer and is recognized as one of "List of Internet pioneers, the fathers of the Internet", sharing this title with TCP/IP co-developer Bob Kahn. He has received honorary degrees ...

Vint Cerf
,
Yogen Dalal Instead of a single "inventor", the Internet The Internet (or internet) is the global system of interconnected s that uses the (TCP/IP) to communicate between networks and devices. It is a ' that consists of private, public, academic ...
, and Carl Sunshine published the
Transmission Control Protocol The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules ...
(TCP) specification, , coining the term
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
as a shorthand for
internetworking Internetworking is the practice of interconnecting multiple computer networks, such that any pair of Host (network), hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting s ...
. * In 1976, John Murphy of
Datapoint Corporation Datapoint Corporation, originally known as Computer Terminal Corporation (CTC), was a computer company based in San Antonio ("Cradle of Freedom") , image_map = Bexar SanAntonio.svg , mapsize = ...
created
ARCNET Attached Resource Computer NETwork (ARCNET or ARCnet) is a communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications sys ...
, a token-passing network first used to share storage devices. * In 1977, the first long-distance fiber network was deployed by GTE in Long Beach, California. * In 1977,
Xerox Network Systems Xerox Network Systems (XNS) is a computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of techno ...
(XNS) was developed by Robert Metcalfe and Yogen Dalal at
Xerox Xerox Holdings Corporation (; also known simply as Xerox) is an American corporation that sells print and digital document An electronic document is any electronic media content (other than computer program A computer program is a collectio ...
. * In 1979, Robert Metcalfe pursued making Ethernet an open standard. * In 1980, Ethernet was upgraded from the original 2.94 Mbit/s protocol to the 10 Mbit/s protocol, which was developed by Ron Crane, Bob Garner, Roy Ogus, and Yogen Dalal. *In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s. By 1998, Ethernet supported transmission speeds of 1 Gbit/s. Subsequently, higher speeds of up to 400 Gbit/s were added (). The scaling of Ethernet has been a contributing factor to its continued use.


Use

A computer network extends interpersonal communications by electronic means with various technologies, such as
email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_dev ...

email
,
instant messaging Instant messaging (IM) technology is a type of online chat Online chat may refer to any kind of communication over the Internet that offers a real-time text, real-time transmission of text-based, text messages from sender to receiver. Chat ...

instant messaging
,
online chat Online chat may refer to any kind of communication Communication (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ar ...
, voice and video telephone calls, and
video conferencing Videotelephony, sometimes also referred to as video teleconference or videoconferencing, comprises the technologies for the reception and transmission of audio Audio most commonly refers to sound In physics Physics (from grc, φυ ...
. A network allows sharing of network and computing resources. Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer or use of a shared storage device. A network allows sharing of files, data, and other types of information giving authorized users the ability to access information stored on other computers on the network.
Distributed computing Distributed computing is a field of computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Comp ...
uses computing resources across a network to accomplish tasks.


Network packet

Most modern computer networks use protocols based on packet-mode transmission. A
network packet In telecommunications and computer networking, a network packet is a formatted unit of Data (computing), data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the ''Payload ...
is a formatted unit of
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...

data
carried by a
packet-switched network In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of ...
. Packets consist of two types of data: control information and user data (payload). The control information provides data the network needs to deliver the user data, for example, source and destination
network address A network address is an identifier for a node In general, a node is a localized swelling (a "knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, ...
es,
error detection In information theory Information theory is the scientific study of the quantification, storage, and communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subjec ...
codes, and sequencing information. Typically, control information is found in packet headers and
trailers Trailer may refer to: Transportation * Trailer (vehicle), an unpowered vehicle pulled by a powered vehicle ** Bicycle trailer, a wheeled frame for hitching to a bicycle to tow cargo or passengers ** Full-trailer ** Semi-trailer **Horse trailer an ...
, with
payload data In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
in between. With packets, the
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of the transmission medium can be better shared among users than if the network were circuit switched. When one user is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link isn't overused. Often the route a packet needs to take through a network is not immediately available. In that case, the packet is queued and waits until a link is free. The physical link technologies of packet network typically limit the size of packets to a certain
maximum transmission unit In computer networking A computer network is a set of computers sharing resources located on or provided by Node (networking), network nodes. The computers use common communication protocols over digital signal, digital Interconnection, inter ...
(MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message.


Network topology

The physical or geographic locations of network nodes and links generally have relatively little effect on a network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks, a single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore most network diagrams are arranged by their
network topology Network topology is the arrangement of the elements (links Link or Links may refer to: Places * Link, West Virginia, an unincorporated community in the US * Link River, Klamath Falls, Oregon, US People with the name * Link (singer) (Linco ...
which is the map of logical interconnections of network hosts. Common layouts are: *
Bus network A bus network is a network topology in which nodes In general, a node is a localized swelling (a "knot") or a point of intersection (a Vertex (graph theory), vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a m ...
: all nodes are connected to a common medium along this medium. This was the layout used in the original
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
, called
10BASE5 10BASE5 (also known as thick Ethernet or thicknet) was the first commercially available variant of . The technology was standardized in 1982 as . 10BASE5 uses a thick and stiff coaxial cable up to in length. Up to 100 stations can be connected ...
and
10BASE2 Different types of T-connectors, with Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ' ...
. This is still a common topology on the
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without r ...
, although modern
physical layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without regard to its underlying internal structure and technolo ...
variants use point-to-point links instead, forming a star or a tree. *
Star network A star network is an implementation of a spoke–hub distribution paradigm in computer networks. In a star network, every Host (network), host is connected to a central Hub (network science), hub. In its simplest form, one central hub acts a ...

Star network
: all nodes are connected to a special central node. This is the typical layout found in a small switched Ethernet LAN, where each client connects to a central network switch, and logically in a
wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more point ...
, where each wireless client associates with the central
wireless access point #REDIRECT Wireless access point #REDIRECT Wireless access point#REDIRECT Wireless access point In computer networking, a wireless access point (WAP), or more generally just access point (AP), is a networking hardware device that allows other ...

wireless access point
. *
Ring network Image showing ring network layout A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node – a ring. Data travels from node to node, with each ...
: each node is connected to its left and right neighbour node, such that all nodes are connected and that each node can reach each other node by traversing nodes left- or rightwards.
Token ring Token Ring network IBM hermaphroditic connector with locking clip. Screen connectors are prominently visible, gold-plated signal connectors less so. Token Ring is a computer network A computer network is a set of s sharing resources lo ...

Token ring
networks, and the
Fiber Distributed Data Interface Dual-attach FDDI board for SBus Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network. It uses optical fiber as its standard underlying physical medium, although it was also later specified to use ...
(FDDI), made use of such a topology. *
Mesh network A mesh network (or simply meshnet) is a local network topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with th ...

Mesh network
: each node is connected to an arbitrary number of neighbours in such a way that there is at least one traversal from any node to any other. *
Fully connected network Network topology is the arrangement of the elements (Data link, links, Node (networking), nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, ...
: each node is connected to every other node in the network. * Tree network: nodes are arranged hierarchically. This is the natural topology for a larger Ethernet network with multiple switches and without redundant meshing. The physical layout of the nodes in a network may not necessarily reflect the network topology. As an example, with FDDI, the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding.


Overlay network

An overlay network is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many peer-to-peer networks are overlay networks. They are organized as nodes of a virtual system of links that run on top of the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
. Overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using
modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telep ...

modem
s before any Telecommunications network#Data network, data network existed. The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the telephone network. Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. Address Resolution Protocol, Address resolution and routing are the means that allow mapping of a fully connected IP overlay network to its underlying network. Another example of an overlay network is a distributed hash table, which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a associative array, map) indexed by keys. Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees achieve higher-quality streaming media. Previous proposals such as IntServ, DiffServ, and IP Multicast have not seen wide acceptance largely because they require modification of all Router (computing), routers in the network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers. The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination. For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast). Academic research includes end system multicast, resilient routing and quality of service studies, among others.


Network links

The transmission media (often referred to in the literature as the ''Transmission medium, physical medium'') used to link devices to form a computer network include electrical cable, optical fiber, and free space. In the OSI model, the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer. A widely adopted ''family'' that uses copper and fiber media in local area network (LAN) technology are collectively known as
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
. The media and protocol standards that enable communication between networked devices over Ethernet are defined by IEEE 802.3. Wireless LAN standards use radio waves, others use IrDA, infrared signals as a transmission medium. Power line communication uses a building's power cabling to transmit data.


Wired

The following classes of wired technologies are used in computer networking. *''Coaxial cable'' is widely used for cable television systems, office buildings, and other work-sites for local area networks. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second. *ITU-T G.hn technology uses existing home wiring (Ethernet over coax, coaxial cable, phone lines and Power line communication, power lines) to create a high-speed local area network. *''Twisted pair'' cabling is used for wired
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
and other standards. It typically consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk (electronics), crosstalk and electromagnetic induction. The transmission speed ranges from 2 Mbit/s to 10 Gbit/s. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several category cable, category ratings, designed for use in various scenarios. *An ''optical fiber'' is a glass fiber. It carries pulses of light that represent data via lasers and Optical amplifier, optical amplifiers. Some advantages of optical fibers over metal wires are very low transmission loss and immunity to electrical interference. Using dense Wavelength-division multiplexing, wave division multiplexing, optical fibers can simultaneously carry multiple streams of data on different wavelengths of light, which greatly increases the rate that data can be sent to up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used for undersea cables to interconnect continents. There are two basic types of fiber optics, single-mode optical fiber (SMF) and multi-mode optical fiber (MMF). Single-mode fiber has the advantage of being able to sustain a coherent signal for dozens or even a hundred kilometers. Multimode fiber is cheaper to terminate but is limited to a few hundred or even only a few dozens of meters, depending on the data rate and cable grade.


Wireless

Network connections can be established wirelessly using radio or other electromagnetic means of communication. *'' Terrestrial microwave'' – Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately apart. *''Communications satellites'' – Satellites also communicate via microwave. The satellites are stationed in space, typically in geosynchronous orbit above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals. *''Cellular networks'' use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area is served by a low-power transceiver. *''Radio and spread spectrum technologies'' – Wireless LANs use a high-frequency radio technology similar to digital cellular. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology known as Wi-Fi. *''Free-space optical communication'' uses visible or invisible light for communications. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices. * Extending the Internet to interplanetary dimensions via radio waves and optical means, the Interplanetary Internet. * IP over Avian Carriers was a humorous April fool's Request for Comments, issued as . It was implemented in real life in 2001. The last two cases have a large round-trip delay time, which gives slow two-way communication but doesn't prevent sending large amounts of information (they can have high throughput).


Network nodes

Apart from any physical transmission media, networks are built from additional basic Systems engineering, system building blocks, such as network interface controllers (NICs), repeaters, Ethernet hub, hubs, Network bridge, bridges, Network switch, switches, Router (computing), routers,
modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telep ...

modem
s, and Firewall (computing), firewalls. Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions.


Network interfaces

A network interface controller (NIC) is computer hardware that connects the computer to the network media and has the ability to process low-level network information. For example, the NIC may have a connector for accepting a cable, or an aerial for wireless transmission and reception, and the associated circuitry. In
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
networks, each network interface controller has a unique MAC address, Media Access Control (MAC) address—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, the Institute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six Octet (computing), octets. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.


Repeaters and hubs

A repeater is an Electronics, electronic device that receives a network signal (information theory), signal, cleans it of unnecessary noise and regenerates it. The signal is retransmission (data networks), retransmitted at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart. Repeaters work on the physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a propagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet 5-4-3 rule. An Ethernet repeater with multiple ports is known as an Ethernet hub. In addition to reconditioning and distributing network signals, a repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches.


Bridges and switches

Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports. Bridges only have two ports but a switch can be thought of as a multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches. Bridges and switches operate at the
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without r ...
(layer 2) of the OSI model and Bridging (networking), bridge traffic between two or more network segments to form a single local network. Both are devices that forward Frame (networking), frames of data between Computer port (hardware), ports based on the destination MAC address in each frame. They learn the association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device Broadcast, unknown unicast and multicast traffic, broadcasts the request to all ports except the source, and discovers the location from the reply. Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down a large, congested network into an aggregation of smaller, more efficient networks.


Routers

A Router (computing), router is an
internetworking Internetworking is the practice of interconnecting multiple computer networks, such that any pair of Host (network), hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting s ...
device that forwards Network packet, packets between networks by processing the addressing or routing information included in the packet. The routing information is often processed in conjunction with the routing table. A router uses its routing table to determine where to forward packets and does not require broadcasting packets which is inefficient for very big networks.


Modems

Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are modulation, modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission. Early modems modulated audio signals sent over a standard voice telephone line. Modems are still commonly used for telephone lines, using a digital subscriber line technology and cable television systems using DOCSIS technology.


Firewalls

A Firewall (computing), firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as the Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyberattack, cyber attacks.


Communication protocols

A
communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networ ...
is a set of rules for exchanging information over a network. Communication protocols have various characteristics. They may be Connection-oriented communication, connection-oriented or Connectionless communication, connectionless, they may use circuit mode or
packet switching In telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or re ...
, and they may use hierarchical addressing or flat addressing. In a protocol stack, often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is HTTP (the World Wide Web protocol) running over transmission control protocol, TCP over Internet protocol, IP (the Internet Protocol Suite, Internet protocols) over IEEE 802.11 (the Wi-Fi protocol). This stack is used between the wireless router and the home user's personal computer when the user is surfing the web. There are many communication protocols, a few of which are described below.


Common protocols


Internet Protocol Suite

The Internet Protocol Suite, also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core, the protocol suite defines the addressing, identification, and routing specifications for IPv4, Internet Protocol Version 4 (IPv4) and for IPv6, the next generation of the protocol with a much enlarged addressing capability. The Internet Protocol Suite is the defining set of protocols for the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
.Andrew S. Tannenbaum, ''Computer Networks'', 4th Edition, Prentice Hall (2003)


IEEE 802

IEEE 802 is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides a diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model. For example, Bridging (networking), MAC bridging (IEEE 802.1D) deals with the routing of Ethernet packets using a Spanning Tree Protocol. IEEE 802.1Q describes Virtual LAN, VLANs, and IEEE 802.1X defines a port-based Network Access Control protocol, which forms the basis for the authentication mechanisms used in VLANs (but it is also found in WLANs) – it is what the home user sees when the user has to enter a "wireless access key".


= Ethernet

=
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
is a family of technologies used in wired LANs. It is described by a set of standards together called IEEE 802.3 published by the Institute of Electrical and Electronics Engineers.


= Wireless LAN

= Wireless LAN based on the IEEE 802.11 standards, also widely known as WLAN or WiFi, is probably the most well-known member of the IEEE 802 protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet.


SONET/SDH

Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from a variety of different sources, primarily to support Circuit switching, circuit-switched digital telephony. However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames.


Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing and encodes data into small, fixed-sized cell relay, cells. This differs from other protocols such as the Internet Protocol Suite or
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
that use variable-sized packets or Ethernet frame, frames. ATM has similarities with both Circuit switching, circuit and Packet switching, packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, Latency (engineering), low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins. While the role of ATM is diminishing in favor of Next generation network, next-generation networks, it still plays a role in the Last mile (telecommunications), last mile, which is the connection between an Internet service provider and the home user.


Cellular standards

There are a number of different digital cellular standards, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne, CDMA2000, Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN).


Routing

Routing is the process of selecting network paths to carry network traffic. Routing is performed for many kinds of networks, including circuit switching networks and packet switched networks. In packet-switched networks, routing protocols direct packet forwarding (the transit of logically addressed
network packet In telecommunications and computer networking, a network packet is a formatted unit of Data (computing), data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the ''Payload ...
s from their source toward their ultimate destination) through intermediate Node (networking), nodes. Intermediate nodes are typically network hardware devices such as Router (computing), routers, Bridging (networking), bridges, gateway (telecommunications), gateways, Firewall (computing), firewalls, or network switch, switches. General-purpose
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

computer
s can also forward packets and perform routing, though they are not specialized hardware and may suffer from the limited performance. The routing process usually directs forwarding on the basis of routing tables, which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the router's Computer storage, memory, is very important for efficient routing. There are usually multiple routes that can be taken, and to choose between them, different elements can be considered to decide which routes get installed into the routing table, such as (sorted by priority): #''Prefix-Length'': where longer subnet masks are preferred (independent if it is within a routing protocol or over a different routing protocol) #''Metric'': where a lower metric/cost is preferred (only valid within one and the same routing protocol) #''Administrative distance'': where a lower distance is preferred (only valid between different routing protocols) Most routing algorithms use only one network path at a time. Multipath routing techniques enable the use of multiple alternative paths. Routing, in a more narrow sense of the term, is often contrasted with bridging (networking), bridging in its assumption that
network address A network address is an identifier for a node In general, a node is a localized swelling (a "knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, ...
es are structured and that similar addresses imply proximity within the network. Structured addresses allow a single routing table entry to represent the route to a group of devices. In large networks, structured addressing (routing, in the narrow sense) outperforms unstructured addressing (bridging). Routing has become the dominant form of addressing on the Internet. Bridging is still widely used within localized environments.


Geographic scale

Networks may be characterized by many properties or features, such as physical capacity, organizational purpose, user authorization, access rights, and others. Another distinct classification method is that of the physical extent or geographic scale. ; Nanoscale network A IEEE P1906.1, nanoscale communication network has key components implemented at the nanoscale including message carriers and leverages physical principles that differ from macroscale communication mechanisms. Nanoscale communication extends communication to very small sensors and actuators such as those found in biological systems and also tends to operate in environments that would be too harsh for classical communication. ; Personal area network A personal area network (PAN) is a computer network used for communication among computers and different information technological devices close to one person. Some examples of devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and even video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters. A wired PAN is usually constructed with USB and FireWire connections while technologies such as Bluetooth and infrared communication typically form a wireless PAN. ;Local area network A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as a home, school, office building, or closely positioned group of buildings. Each computer or device on the network is a node (networking), node. Wired LANs are most likely based on
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
technology. Newer standards such as ITU-T G.hn also provide a way to create a wired LAN using existing wiring, such as coaxial cables, telephone lines, and power lines. The defining characteristics of a LAN, in contrast to a wide area network (WAN), include higher bandwidth (computing), data transfer rates, limited geographic range, and lack of reliance on leased lines to provide connectivity. Current Ethernet or other IEEE 802.3 LAN technologies operate at data transfer rates up to 100 Gigabit Ethernet, 100 Gbit/s, standardized by IEEE in 2010. Currently, Terabit Ethernet, 400 Gbit/s Ethernet is being developed. A LAN can be connected to a WAN using a router (computing), router. ;Home area network A home area network (HAN) is a residential LAN used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a cable TV or digital subscriber line (DSL) provider. ;Storage area network A storage area network (SAN) is a dedicated network that provides access to consolidated, block-level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the local area network by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium-sized business environments. ;Campus area network A campus area network (CAN) is made up of an interconnection of LANs within a limited geographical area. The networking equipment (switches, routers) and transmission media (optical fiber, copper plant, Category 5 cable, Cat5 cabling, etc.) are almost entirely owned by the campus tenant/owner (an enterprise, university, government, etc.). For example, a university campus network is likely to link a variety of campus buildings to connect academic colleges or departments, the library, and student residence halls. ;Backbone network A backbone network is part of a computer network infrastructure that provides a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks within the same building, across different buildings, or over a wide area. For example, a large company might implement a backbone network to connect departments that are located around the world. The equipment that ties together the departmental networks constitutes the network backbone. When designing a network backbone, Network performance management, network performance and network congestion are critical factors to take into account. Normally, the backbone network's capacity is greater than that of the individual networks connected to it. Another example of a backbone network is the Internet backbone, which is a massive, global system of fiber-optic cable and optical networking that carry the bulk of data between Wide area network, wide area networks (WANs), metro, regional, national and transoceanic networks. ;Metropolitan area network A metropolitan area network (MAN) is a large computer network that usually spans a city or a large campus. ;Wide area network A wide area network (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances. A WAN uses a communications channel that combines many types of media such as telephone lines, cables, and airwaves. A WAN often makes use of transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI model, OSI reference model: the physical layer, the
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without r ...
, and the network layer. ;Enterprise private network An enterprise private network is a network that a single organization builds to interconnect its office locations (e.g., production sites, head offices, remote offices, shops) so they can share computer resources. ;Virtual private network A virtual private network (VPN) is an overlay network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said to be tunneled through the larger network when this is the case. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features. VPN may have best-effort performance or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point. ;Global area network A global area network (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial
wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more point ...
s.


Organizational scope

Networks are typically managed by the organizations that own them. Private enterprise networks may use a combination of intranets and extranets. They may also provide network access to the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
, which has no single owner and permits virtually unlimited global connectivity.


Intranet

An intranet is a set of networks that are under the control of a single administrative entity. The intranet uses the IP address, IP protocol and IP-based tools such as web browsers and file transfer applications. The administrative entity limits the use of the intranet to its authorized users. Most commonly, an intranet is the internal LAN of an organization. A large intranet typically has at least one web server to provide users with organizational information. An intranet is also anything behind the router on a local area network.


Extranet

An extranet is a network that is also under the administrative control of a single organization but supports a limited connection to a specific external network. For example, an organization may provide access to some aspects of its intranet to share data with its business partners or customers. These other entities are not necessarily trusted from a security standpoint. Network connection to an extranet is often, but not always, implemented via WAN technology.


Internet

An internetwork is the connection of multiple different types of computer networks to form a single computer network by layering on top of the different networking software and connecting them together using routers. The
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
is the largest example of internetwork. It is a global system of interconnected governmental, academic, corporate, public, and private computer networks. It is based on the networking technologies of the Internet Protocol Suite. It is the successor of the ARPANET, Advanced Research Projects Agency Network (ARPANET) developed by Defense Advanced Research Projects Agency, DARPA of the United States Department of Defense. The Internet utilizes copper communications and the optical networking backbone to enable the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system An information system (IS) is a formal, sociotechnical Sociotechnical systems (STS) in organizational development is an approach to complex organizational ...
(WWW), the Internet of things, Internet of Things, video transfer, and a broad range of information services. Participants on the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite and an addressing system (IP addresses) administered by the Internet Assigned Numbers Authority and regional Internet registry, address registries. Service providers and large enterprises exchange information about the routing, reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.


Darknet

A darknet is an overlay network, typically running on the Internet, that is only accessible through specialized software. A darknet is an anonymizing network where connections are made only between trusted peers — sometimes called "friends" (Friend-to-friend, F2F) — using non-standard Communications protocol, protocols and Port (computer networking), ports. Darknets are distinct from other distributed peer-to-peer networks as Peer-to-peer file sharing, sharing is anonymous (that is, IP addresses are not publicly shared), and therefore users can communicate with little fear of governmental or corporate interference.


Network service

Network services are applications hosted by servers on a computer network, to Service (systems architecture), provide some functionality for members or users of the network, or to help the network itself to operate. The
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system An information system (IS) is a formal, sociotechnical Sociotechnical systems (STS) in organizational development is an approach to complex organizational ...
, E-mail, printing and distributed file system, network file sharing are examples of well-known network services. Network services such as DNS (Domain Name System) give names for Internet protocol, IP and MAC addresses (people remember names like “nm.lan” better than numbers like “210.121.67.18”),, "Domain names – Implementation and Specification", P. Mockapetris (November 1987) and Dynamic Host Configuration Protocol, DHCP to ensure that the equipment on the network has a valid IP address. Services are usually based on a protocol (computing), service protocol that defines the format and sequencing of messages between clients and servers of that network service.


Network performance


Bandwidth

Bandwidth in bit/s may refer to consumed bandwidth, corresponding to achieved throughput or goodput, i.e., the average rate of successful data transfer through a communication path. The throughput is affected by technologies such as bandwidth shaping, bandwidth management, bandwidth throttling, bandwidth cap, bandwidth allocation (for example bandwidth allocation protocol and dynamic bandwidth allocation), etc. A bit stream's bandwidth is proportional to the average consumed signal bandwidth in hertz (the average spectral bandwidth of the analog signal representing the bit stream) during a studied time interval.


Network delay

''Network delay'' is a design and performance characteristic of a telecommunications network. It specifies the Latency (engineering), latency for a bit of data to travel across the network from one communication endpoint to another. It is typically measured in multiples or fractions of a second. Delay may differ slightly, depending on the location of the specific pair of communicating endpoints. Engineers usually report both the maximum and average delay, and they divide the delay into several parts: * Processing delay time it takes a router to process the packet header * Queuing delay time the packet spends in routing queues * Transmission delay time it takes to push the packet's bits onto the link * Propagation delay time for a signal to propagate through the media A certain minimum level of delay is experienced by signals due to the time it takes to Data transmission, transmit a packet serially through a Data link, link. This delay is extended by more variable levels of delay due to network congestion. IP network delays can range from a few milliseconds to several hundred milliseconds.


Quality of service

Depending on the installation requirements, network performance is usually measured by the quality of service of a telecommunications product. The parameters that affect this typically can include throughput, jitter, bit error rate and latency (engineering), latency. The following list gives examples of network performance measures for a circuit-switched network and one type of
packet-switched network In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of ...
, viz. ATM: *Circuit-switched networks: In circuit switched networks, network performance is synonymous with the grade of service. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads. Other types of performance measures can include the level of noise and echo. *ATM: In an Asynchronous Transfer Mode (ATM) network, performance can be measured by line rate, quality of service (QoS), data throughput, connect time, stability, technology, modulation technique, and modem enhancements. There are many ways to measure the performance of a network, as each network is different in nature and design. Performance can also be modeled instead of measured. For example, state diagram, state transition diagrams are often used to model queuing performance in a circuit-switched network. The network planner uses these diagrams to analyze how the network performs in each state, ensuring that the network is optimally designed.


Network congestion

Network congestion occurs when a link or node is subjected to a greater data load than it is rated for, resulting in a deterioration of its quality of service. When networks are congested and queues become too full, packets have to be discarded, and so networks rely on re-transmission. Typical effects of congestion include queueing delay, packet loss or the blocking probability, blocking of new connections. A consequence of these latter two is that incremental increases in offered load lead either to only a small increase in the network throughput or to a reduction in network throughput. Network protocols that use aggressive Retransmission (data networks), retransmissions to compensate for packet loss tend to keep systems in a state of network congestion—even after the initial load is reduced to a level that would not normally induce network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as ''congestive collapse''. Modern networks use congestion control, congestion avoidance and
traffic control Traffic management is a key branch within logistics Logistics is generally the detailed organization and implementation of a complex operation. In a general business sense, logistics is the management of the flow of things between the point of ...
techniques to try to avoid congestion collapse (i.e. endpoints typically slow down or sometimes even stop transmission entirely when the network is congested). These techniques include: exponential backoff in protocols such as 802.11's Carrier sense multiple access with collision avoidance, CSMA/CA and the original
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
, sliding window, window reduction in Transmission control protocol, TCP, and fair queueing in devices such as Router (computing), routers. Another method to avoid the negative effects of network congestion is implementing priority schemes so that some packets are transmitted with higher priority than others. Priority schemes do not solve network congestion by themselves, but they help to alleviate the effects of congestion for some services. An example of this is 802.1p. A third method to avoid network congestion is the explicit allocation of network resources to specific flows. One example of this is the use of Contention-Free Transmission Opportunities (CFTXOPs) in the ITU-T G.hn standard, which provides high-speed (up to 1 Gbit/s) Local area networking over existing home wires (power lines, phone lines and coaxial cables). For the Internet, addresses the subject of congestion control in detail.


Network resilience

resilience (network), Network resilience is "the ability to provide and maintain an acceptable level of Service (systems architecture), service in the face of Fault (technology), faults and challenges to normal operation.”


Security

Computer networks are also used by security hackers to deploy computer viruses or computer worms on devices connected to the network, or to prevent these devices from accessing the network via a denial-of-service attack.


Network security

Network Security consists of provisions and policies adopted by the network administrator to prevent and monitor unauthorized access, misuse, modification, or denial of the computer network and its network-accessible resources. Network security is the authorization of access to data in a network, which is controlled by the network administrator. Users are assigned an ID and password that allows them access to information and programs within their authority. Network security is used on a variety of computer networks, both public and private, to secure daily transactions and communications among businesses, government agencies, and individuals.


Network surveillance

Network surveillance is the monitoring of data being transferred over computer networks such as the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
. The monitoring is often done surreptitiously and may be done by or at the behest of governments, by corporations, criminal organizations, or individuals. It may or may not be legal and may or may not require authorization from a court or other independent agency. Computer and network surveillance programs are widespread today, and almost all Internet traffic is or could potentially be monitored for clues to illegal activity. Surveillance is very useful to governments and law enforcement to maintain social control, recognize and monitor threats, and prevent/investigate criminal activity. With the advent of programs such as the Total Information Awareness program, technologies such as NarusInsight, high-speed surveillance computers and Surveillance#Biometric, biometrics software, and laws such as the Communications Assistance For Law Enforcement Act, governments now possess an unprecedented ability to monitor the activities of citizens. However, many civil rights and privacy groups—such as Reporters Without Borders, the Electronic Frontier Foundation, and the American Civil Liberties Union—have expressed concern that increasing surveillance of citizens may lead to a mass surveillance society, with limited political and personal freedoms. Fears such as this have led to numerous lawsuits such as ''Hepting v. AT&T''. The hacktivist group Anonymous (group), Anonymous has hacked into government websites in protest of what it considers "draconian surveillance".


End to end encryption

End-to-end encryption (E2EE) is a digital communications paradigm of uninterrupted protection of data traveling between two communicating parties. It involves the originating party encryption, encrypting data so only the intended recipient can decrypt it, with no dependency on third parties. End-to-end encryption prevents intermediaries, such as Internet providers or application service providers, from discovering or tampering with communications. End-to-end encryption generally protects both confidentiality and data integrity, integrity. Examples of end-to-end encryption include HTTPS for web traffic, Pretty Good Privacy, PGP for
email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_dev ...

email
, Off-the-Record Messaging, OTR for
instant messaging Instant messaging (IM) technology is a type of online chat Online chat may refer to any kind of communication over the Internet that offers a real-time text, real-time transmission of text-based, text messages from sender to receiver. Chat ...

instant messaging
, ZRTP for telephony, and Terrestrial Trunked Radio, TETRA for radio. Typical Server (computing), server-based communications systems do not include end-to-end encryption. These systems can only guarantee the protection of communications between Client (computing), clients and servers, not between the communicating parties themselves. Examples of non-E2EE systems are Google Talk, Yahoo Messenger, Facebook, and Dropbox (service), Dropbox. Some such systems, for example, LavaBit and SecretInk, have even described themselves as offering "end-to-end" encryption when they do not. Some systems that normally offer end-to-end encryption have turned out to contain a Backdoor (computing), back door that subverts negotiation of the encryption key between the communicating parties, for example Skype or Hushmail. The end-to-end encryption paradigm does not directly address risks at the endpoints of the communication themselves, such as the Exploit (computer security), technical exploitation of Client (computing), clients, poor quality random number generators, or key escrow. E2EE also does not address traffic analysis, which relates to things such as the identities of the endpoints and the times and quantities of messages that are sent.


SSL/TLS

The introduction and rapid growth of e-commerce on the World Wide Web in the mid-1990s made it obvious that some form of authentication and encryption was needed. Netscape Corporation, Netscape took the first shot at a new standard. At the time, the dominant web browser was Netscape Navigator. Netscape created a standard called secure socket layer (SSL). SSL requires a server with a certificate. When a client requests access to an SSL-secured server, the server sends a copy of the certificate to the client. The SSL client checks this certificate (all web browsers come with an exhaustive list of CA root certificates preloaded), and if the certificate checks out, the server is authenticated and the client negotiates a symmetric-key cipher for use in the session. The session is now in a very secure encrypted tunnel between the SSL server and the SSL client.


Views of networks

Users and network administrators typically have different views of their networks. Users can share printers and some servers from a workgroup, which usually means they are in the same geographic location and are on the same LAN, whereas a Network Administrator is responsible to keep that network up and running. A community-of-interest network, community of interest has less of a connection of being in a local area and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via peer-to-peer technologies. Network administrators can see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., Router (computing), routers, Network bridge, bridges and Application-level gateway, application layer gateways) that interconnect via the transmission media. Logical networks, called, in the TCP/IP architecture, subnetwork, subnets, map onto one or more transmission media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using Virtual LAN, virtual LAN (VLAN) technology. Both users and administrators are aware, to varying extents, of the trust and scope characteristics of a network. Again using TCP/IP architectural terminology, an intranet is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees)., "BGP/MPLS VPNs", E. Rosen; Y. Rekhter (March 1999) Intranets do not have to be connected to the Internet, but generally have a limited connection. An extranet is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers). Unofficially, the Internet is the set of users, enterprises, and content providers that are interconnected by Internet Service Providers (ISP). From an engineering viewpoint, the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
is the set of subnets, and aggregates of subnets, that share the registered IP address space and exchange information about the reachability of those IP addresses using the Border Gateway Protocol. Typically, the human-readable names of servers are translated to IP addresses, transparently to users, via the directory function of the Domain Name System (DNS). Over the Internet, there can be Business-to-business, business-to-business (B2B), Business-to-consumer, business-to-consumer (B2C) and Consumer-to-consumer electronic commerce, consumer-to-consumer (C2C) communications. When money or sensitive information is exchanged, the communications are apt to be protected by some form of communications security mechanism. Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users and administrators, using secure Virtual Private Network (VPN) technology.


Journals and newsletters


Open Computer Science
(Open access (publishing), open access journal)


See also

*Comparison of network diagram software *Cyberspace *History of the Internet *Information Age *Information revolution * *Minimum-Pairs Protocol *Network simulation *Network planning and design *Network traffic control


References


Further reading

* Shelly, Gary, et al. "Discovering Computers" 2003 Edition. * Wendell Odom, Rus Healy, Denise Donohue. (2010) CCIE Routing and Switching. Indianapolis, IN: Cisco Press * Kurose James F and Keith W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet, Pearson Education 2005. * William Stallings, ''Computer Networking with Internet Protocols and Technology'', Pearson Education 2004. * List of important publications in computer science#Computer networks, Important publications in computer networks * Network Communication Architecture and Protocols: OSI Network Architecture 7 Layers Model * Dimitri Bertsekas, and Robert Gallager, "Data Networks," Prentice Hall, 1992.


External links

*
IEEE Ethernet manufacturer information

A computer networking acronym guide
{{Authority control Computer networks,