In the

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...

(DNS) hierarchy, a subdomain is a domain that is a part of another (main) domain. For example, if a domain offered an online store as part of their website example.com, it might use the subdomain shop.example.com .

Overview

The

(DNS) has a

tree structure A tree structure, tree diagram, or tree model is a way of representing the hierarchical nature of a structure in a graphical form. It is named a "tree structure" because the classic representation resembles a tree, although the chart is genera ...

or hierarchy, which includes nodes on the tree being a

domain name A domain name is a string that identifies a realm of administrative autonomy, authority or control within the Internet. Domain names are often used to identify services provided through the Internet, such as websites, email services and more. As ...

. A subdomain is a domain that is part of a larger domain. Each label may contain from 1 to 63 octets. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.RFC 1035, ''Domain names--Implementation and specification'', P. Mockapetris (Nov 1987) Subdomains are defined by editing the DNS zone file pertaining to the parent domain. However, there is an ongoing debate over the use of the term "subdomain" when referring to names which map to the Address record A (host) and various other types of zone records which may map to any public

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

destination and any type of server. Network Operations teams insist that it is inappropriate to use the term "subdomain" to refer to any mapping other than that provided by zone NS (name server) records and any server-destination other than that. According t
RFC 1034
''"a domain is a subdomain of another domain if it is contained within that domain"''. Based on that definition, a host cannot be a subdomain, only a domain can be a subdomain. A subdomain will also have a separate zone file with a SOA record (Start of Authority). Most domain registries only allocate a two-level domain name. Hosting services typically provide DNS Servers to resolve subdomains within that master domain. Subdomain-en

A fully qualified domain name consists of multiple parts. For example, take the English Wikipedia domain en.wikipedia.org. The en is a subdomain of wikipedia.org. Although wikipedia.org is usually considered to be the

, wikipedia is actually a sub-domain of the org

TLD A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the root zone of the name space. For all domains in ...

(top level domain). Any fully qualified domain name can be a host or a subdomain. A domain name that does not include any subdomains is known as an ''apex domain'', ''root domain'', or ''bare domain''. For example, wikipedia.org is the apex domain of Wikipedia, which redirects to the subdomain www.wikipedia.org.

Subdomain usage

Subdomains are often used by internet service providers supplying web services. They allocate one (or more) subdomains to their clients who do not have their own domain name. This allows independent administration by the clients over their subdomain. Subdomains are also used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as www.cs.example.edu. There are some widely recognized subdomains such as

WWW The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web se ...

and

FTP The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...

. This allows for a structure where the domain contains administrative directories and files including the FTP directories and webpages. The FTP subdomain could contain logs and the web page directories, while the WWW subdomain contains the directories for the webpages. Independent authentication for each domain provides access control over the various levels of the domain.

Uses

United Kingdom

In the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...

, the second-level domain names are standard and branch off from the

top-level domain A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the root zone of the name space. For all domains in ...

. For example: * .ac.uk -

academic An academy (Attic Greek: Ἀκαδήμεια; Koine Greek Ἀκαδημία) is an institution of secondary or tertiary higher learning (and generally also research or honorary membership). The name traces back to Plato's school of philosophy, ...

(

tertiary education Tertiary education, also referred to as third-level, third-stage or post-secondary education, is the educational level following the completion of secondary education. The World Bank, for example, defines tertiary education as including univers ...

further education colleges Further education (often abbreviated FE) in the United Kingdom and Ireland is education in addition to that received at secondary school, that is distinct from the higher education (HE) offered in universities and other academic institutions. It ...

and research establishments) and

learned societies A learned society (; also learned academy, scholarly society, or academic association) is an organization that exists to promote an academic discipline, profession, or a group of related disciplines such as the arts and science. Membership may ...

* .co.uk - general use (usually

commercial Commercial may refer to: * a dose of advertising conveyed through media (such as - for example - radio or television) ** Radio advertisement ** Television advertisement * (adjective for:) commerce, a system of voluntary exchange of products and s ...

) * .gov.uk -

government A government is the system or group of people governing an organized community, generally a state. In the case of its broad associative definition, government normally consists of legislature, executive, and judiciary. Government is ...

(central and local) * .judiciary.uk -

court A court is any person or institution, often as a government institution, with the authority to adjudicate legal disputes between parties and carry out the administration of justice in civil, criminal, and administrative matters in acco ...

s (to be introduced in the near future) * .ltd.uk - limited companies * .me.uk - general use (usually

person A person ( : people) is a being that has certain capacities or attributes such as reason, morality, consciousness or self-consciousness, and being a part of a culturally established form of social relations such as kinship, ownership of prope ...

al) * .mod.uk - Ministry of Defence and

HM Forces The British Armed Forces, also known as His Majesty's Armed Forces, are the military forces responsible for the defence of the United Kingdom, its Overseas Territories and the Crown Dependencies. They also promote the UK's wider interests, su ...

public sites * .net.uk -

ISP An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise private ...

s and network companies (unlike .net, use is restricted to these users) * .nhs.uk -

National Health Service The National Health Service (NHS) is the umbrella term for the publicly funded healthcare systems of the United Kingdom (UK). Since 1948, they have been funded out of general taxation. There are three systems which are referred to using the " ...

institutions * .nic.uk
network use only
(

Nominet UK Nominet UK is currently delegated by IANA to be the manager of the .uk domain name. Nominet directly manages registrations directly under .uk, and some of the second level domains .co.uk, .org.uk, .sch.uk, .me.uk, .net.uk, .ltd.uk and .plc.uk. ...

) * .org.uk - general use (usually for

non-profit organisation A nonprofit organization (NPO) or non-profit organisation, also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized and operated for a collective, public or social benefit, in co ...

s) * .parliament.uk - parliamentary use (only for the

UK Parliament The Parliament of the United Kingdom is the supreme legislative body of the United Kingdom, the Crown Dependencies and the British Overseas Territories. It meets at the Palace of Westminster, London. It alone possesses legislative suprema ...

and the Scottish Parliament) * .plc.uk - public limited companies * .police.uk -

police forces The police are a constituted body of persons empowered by a state, with the aim to enforce the law, to ensure the safety, health and possessions of citizens, and to prevent crime and civil disorder. Their lawful powers include arrest and t ...

* .sch.uk -

Local Education Authorities Local education authorities (LEAs) were local councils in England that are responsible for education within their jurisdiction. The term was used to identify which council (district or county) is locally responsible for education in a system wit ...

school A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes comp ...

s, primary and

secondary education Secondary education or post-primary education covers two phases on the International Standard Classification of Education scale. Level 2 or lower secondary education (less commonly junior secondary education) is considered the second and final pha ...

, community education

Vanity domain

vanity domain In the Domain Name System (DNS), a vanity domain is a domain name whose purpose is to express the individuality of the person on whose behalf it is registered. This contrasts with domain names which resolve to an organisation (e.g. a company) or ...

is a subdomain of an ISP's domain that is aliased to an individual user account, or a subdomain that expresses the individuality of the person on whose behalf it is registered.

Server cluster

Depending on application, a record inside a domain, or subdomain might refer to a

hostname In computer networking, a hostname (archaically nodename) is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication, such as the World Wide Web. Hos ...

, or a service provided by a number of machines in a cluster. Some websites use different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2 etc.

Subdomains versus directories

Subdomains are different from directories. Directories are physical folders on an actual computer, while subdomains are a part of the URL that can be routed to any file or folder on the server machine.

References

{{reflist Domain Name System