An IP header is
header information at the beginning of an
Internet Protocol (IP) packet. An IP packet is the smallest message entity exchanged via the Internet Protocol across an
IP network
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...
. IP packets consist of a header for addressing and routing and a
payload for user data. The header contains information about IP version, source
IP address
An Internet Protocol address (IP address) is a numerical label such as that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...
, destination IP address,
time-to-live, etc. The payload of an IP packet is typically a
datagram
A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The de ...
or segment of the higher-level
transport layer
In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end c ...
protocol, but may be data for an
internet layer
The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport network packets from the originating host across network boundaries; if necessary, to the desti ...
(e.g.,
ICMP or
ICMPv6) or
link layer (e.g.,
OSPF) instead.
Two different versions of IP are used in practice today: IPv4 and IPv6. The IPv6 header uses
IPv6 addresses and thus offers a much bigger
address space but is not backward compatible with IPv4.
IPv4
IPv4
Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. ...
is the fourth version in the development of the
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
IP ...
, and routes most traffic on the
Internet
The Internet (or internet) is the Global network, 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 ...
.
The IPv4 header includes thirteen mandatory fields and is as small as 20 bytes. A fourteenth optional and infrequently used ''options'' field can increase the header size.
IPv6
IPv6
Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communication protocol, communications protocol that provides an identification and location system for computers on networks and routes traffic ...
is the successor to IPv4 and has a different header layout. It was defined in 1998 and is in various stages of
production deployment. The header in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers.
References
{{reflist
Internet Protocol