Fully qualified domain name
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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 domain. Technically, a FQDN has a trailing dot (for example: somehost.example.com.), but most DNS resolvers will treat any domain name that already has a dot as being an FQDN[1] 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. In other cases, such as DNS zone files, the trailing dot is always required to denote a FQDN. 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.
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- ^ Note: On Unix-like systems, this is controlled by the ndots option in the resolv.conf configuration file. The default is one dot implying a FQDN, but it can be changed to any number. Security issues with not interpreting a domain name with a dot as a FQDN are discussed in RFC 1535.
