Fully Qualified Domain Name
A fully qualified domain name (FQDN), sometimes also referred as an absolute domain name,[1] is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone.[2] A fully qualified domain name is distinguished by its lack of ambiguity: it can only be interpreted one way. FQDNs first arose out of the need for uniformity as the Internet was quickly growing in size in the late 1980s.[3]
For example, given a device with a local hostname myhost and a parent domain name example.com, the fully qualified domain name is myhost.example.com. The FQDN therefore uniquely identifies the device —while there may be many hosts in the world called myhost, there can only be one myhost.example.com. In the Domain Name System, and most notably, in DNS zone files, a fully qualified domain name is specified with a trailing dot. For example,
somehost.example.com.
specifies an absolute domain name that ends with an empty top level domain label.[4]
The DNS root domain is unnamed, which is expressed by an empty label, resulting in a domain name ending with the dot separator. However, many DNS resolvers process a domain name that contains a dot in any position as being fully qualified[note 1] or add the final dot needed for the root of the DNS tree. Resolvers process a domain name without a dot as unqualified and automatically append the system's default domain name and the final dot.
Some applications, such as web browsers, try to resolve the domain name part of a Uniform Resource Locator (URL) if the resolver cannot find the specified domain or if it is clearly not fully qualified by appending frequently used top-level domains and testing the result. Some applications, however, never use trailing dots to indicate absoluteness, because the underlying protocols require the use of FQDNs, such as Simple Mail Transfer Protocol (SMTP, an e-mail protocol).[5]