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In the Domain Name System (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 Domain Name System (DNS) has a tree structure 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 Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compo ...
. 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. 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
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 ...
, wikipedia is actually a sub-domain of the org TLD (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 ...
and FTP. 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 second-level domain names are standard and branch off from the top-level domain. 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 univer ...
, further education colleges and research establishments) and learned societies * .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 (central and local) * .judiciary.uk - courts (to be introduced in the near future) * .ltd.uk -
limited companies In a limited company, the liability of members or subscribers of the company is limited to what they have invested or guaranteed to the company. Limited companies may be limited by shares or by guarantee. In a company limited by shares, the liab ...
* .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 propert ...
al) * .mod.uk - Ministry of Defence and HM Forces 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 institutions * .nic.uk
network use only
( Nominet UK) * .org.uk - general use (usually for non-profit organisations) * .parliament.uk - parliamentary use (only for the UK Parliament and the
Scottish Parliament The Scottish Parliament ( gd, Pàrlamaid na h-Alba ; sco, Scots Pairlament) is the devolution in the United Kingdom, devolved, unicameralism, unicameral legislature of Scotland. Located in the Holyrood, Edinburgh, Holyrood area of the capital ...
) * .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 th ...
* .sch.uk - Local Education Authorities, schools, primary and secondary education, community education


Vanity domain

A
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) o ...
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 cluster A computer cluster is a set of computers that work together so that they can be viewed as a single system. Unlike grid computers, computer clusters have each node set to perform the same task, controlled and scheduled by software. The compo ...
s. 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.


See also

*
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 ...
*
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 ...
* Subdirectory *
Subpage A subpage usually refers to a lower level web page in a website or wiki. Example website In this example website, news, about, portfolio and contact are subpages of home. Also, study one and study two are subpages of portfolio. Wikis ...
*
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) o ...
* Webpage


References

{{reflist Domain Name System