The Domain Name System (DNS) has a tree structure or hierarchy, with each non-RR (resource record) node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain. For example,
east.example.com are subdomains of the
example.com domain, which in turn is a subdomain of the
com top-level domain (TLD). A "subdomain" expresses relative dependence, not absolute dependence: for example,
wikipedia.org comprises a subdomain of the
org domain, and
en.wikipedia.org comprises a subdomain of the domain
wikipedia.org. Each label may contain from 1 to 63 octets. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a dot. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation. Thus, when using a single character per label, the limit is 127 levels: 127 characters plus 126 dots have a total length of 253. In practice, some domain registries may have shorter limits.
Subdomains in this context 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 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 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). Merely looking at a URL will not tell you if the left most node is a host or a subdomain.
- .ac.uk - academic (tertiary education, further education colleges and research establishments) and learned societies
- .co.uk - general use (usually commercial)
- .gov.uk - government (central and local)
- .judiciary.uk - courts (to be introduced in the near future)
- .ltd.uk - limited companies
- .me.uk - general use (usually personal)
- .mod.uk - Ministry of Defence and HM Forces public sites
- .net.uk - ISPs 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)
- .plc.uk - public limited companies
- .police.uk - police forces
- .sch.uk - Local Education Authorities, schools, primary and secondary education, community education
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
A vanity domain 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.
Depending on application, a record inside a domain, or subdomain might refer to a hostname, 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..
Some domains host their nameservers as ns1.example.com, ns2.example.com, etc., and these do not typically show up in search engine results.
Subdomains versus directories
- P. Mockapetris (November 1987). "Name space specifications and terminology". Domain names - concepts and facilities. IETF. sec. 3.1. doi:10.17487/RFC1034. RFC 1034. Retrieved 2008-08-03.
- RFC 1035, Domain names--Implementation and specification, P. Mockapetris (Nov 1987)
- "BBC News - UK court systems set to adopt judiciary.uk domain names". BBC News. 23 November 2011. Retrieved 26 February 2014.