A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name, 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. A fully qualified domain name is distinguished by its lack of ambiguity: it can be interpreted only in one way.
The DNS root domain is unnamed, which is expressed by the empty label, resulting in a fully qualified domain name ending with the dot character.
In contrast to a domain name that is fully specified, a domain name that does not include the full path of labels up to the DNS root, is often called a partially qualified domain name.
Example
A device with the hostname myhost and the parent domain example.com has the fully qualified domain name myhost.example.com. The FQDN uniquely distinguishes the device from any other hosts called myhost in other domains.
Syntax
A fully qualified domain name consists of a list of domain labels representing the hierarchy from the lowest relevant level in the DNS to the top-level domain (TLD). The domain labels are concatenated using the full stop (dot, period) character as separator between labels.
The DNS root is unnamed, expressed as the empty label terminated by the dot. This is most notable in DNS zone files in which a fully qualified domain name must be specified with a trailing dot. For example, somehost.example.com. explicitly specifies an absolute domain name that ends with the empty top level domain label.
Resolution
Many DNS resolvers process a domain name that contains a dot in any position as being fully qualified 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 the Simple Mail Transfer Protocol (SMTP).
Partially qualified domain name
A partially qualified domain name does not include all labels to the DNS root. Such a name is also known as a relative domain name, Relative domain names are often simply hostnames, i.e. the left-most label in a fully qualified name.
The portion of a fully qualified domain name that completes a partially qualified domain name is sometimes referred to as a domain suffix.
