Fully qualified domain name
A fully qualified domain name (or FQDN) is an unambiguous domain name that specifies the exact location in the Domain Name System's tree hierarchy through to a top-level domain and finally to the root nameserver. Technically, a FQDN has a trailing dot, example: somehost.example.com., but most DNS resolvers will treat any domain name that already have a dot as being an FQDN and add the final dot needed for the root of the DNS tree. Resolvers will treat a domain name without a dot as unqualified and automatically add a default domain name and the final dot. Some applications, such as web browsers will try to qualify the domain name part of a Uniform Resource Locator (URL) if the resolver can not find the domain. An FQDN differs from a regular domain name by its absoluteness; a default domain name will not be added.
For example, given a device with a local hostname of "myhost" and a default parent domain name of "example.com", the fully qualified domain name is "myhost.example.com.". It therefore uniquely defines the device — whilst there might be many hosts in the world called "myhost", there can only be one "myhost.example.com.".
Notice that there is a dot at the very end of the domain name, i.e. it ends ".com." and not ".com" — this ensures that the name is an FQDN. For example "myhost.bar.com" could be ambiguous, because not all resolvers assume that domain names containing a dot are absolute and the domain name could be the prefix of a longer domain name such as "myhost.bar.com.au", whereas "myhost.bar.com." is a fully qualified domain name. In practice, the trailing dot is almost always omitted in everyday applications, making such domain references technically ambiguous.
See also
External links
- RFC 1035: Domain names: implementation and specification
- RFC 1123: Requirements for Internet Hosts - application and support
- RFC 2181: Clarifications to the DNS specification