Subdomain

For a protein domain inside of a larger domain, seeProtein domain.

In theDomain Name System(DNS) hierarchy, asubdomainis adomainthat is a part of another (main) domain.For example, if a domain offered an online store as part of their websiteexample,it might use the subdomainshop.example.

Overview

TheDomain Name System(DNS) has atree structureor hierarchy, which includes nodes on the tree being adomain name.A subdomain is a domain that is part of a larger domain. Each label may contain from 0 to 63octets.The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.

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 theAddress recordA (host) and various other types of zone records which may map to any publicIP addressdestination 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 zoneNS(name server) records and any server-destination other than that.

According to 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).

Mostdomain registriesonly allocate a two-level domain name. Hosting services typically provide DNS Servers to resolve subdomains within that master domain.

Example of subdomain

A fully qualified domain name consists of multiple parts. For example, take the English Wikipedia domainen.wikipedia.org. Theenis a subdomain ofwikipedia.org. Althoughwikipedia.orgis usually considered to be thedomain name,wikipediais actually a sub-domain of theorgTLD(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 anapex domain,root domain,orbare domain.For example,wikipedia.orgis the apex domain of Wikipedia, which redirects to the subdomainwikipedia.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 ascs.example.edu.

There are some widely recognized subdomains such asWWWandFTP.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 theUnited Kingdom,the second-level domain names are standard and branch off from thetop-level domain.For example:

• .ac.uk:academic(tertiary education,further education collegesandresearch establishments) andlearned societies
• .co.uk: general use (usuallycommercial)
• .gov.uk:government(central and local)
• .judiciary.uk:courts(to be introduced in the near future)
• .ltd.uk:limited companies
• .me.uk: general use (usuallypersonal)
• .mod.uk:Ministry of DefenceandHM Forcespublic sites
• .net.uk:ISPsandnetwork companies(unlike.net,use is restricted to these users)
• .nhs.uk:National Health Serviceinstitutions
• .nic.uk:network use only(Nominet UK)
• .org.uk: general use (usually fornon-profit organisations)
• .parliament.uk: parliamentary use (only for theUK Parliamentand theScottish Parliament)
• .plc.uk:public limited companies
• .police.uk:police forces
• .sch.uk:Local Education Authorities,schools,primaryandsecondary education,community education

Vanity domain

Avanity domainis 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

"WWW2" redirects here. Not to be confused withWW2.

Depending on application, arecordinside a domain, or subdomain might refer to ahostname,or a service provided by a number of machines in a cluster. Some websites use different subdomains to point to differentserver clusters.For example,examplepoints to Server Cluster 1 or Datacentre 1, andwww2.examplepoints 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 theURLthat can be routed to any file or folder on the server machine.

See also

• Domain name
• Hostname
• Subdirectory
• Subpage
• Vanity domain
• Webpage