Fully qualified domain name
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 only be interpreted 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.
The need for fully qualified domain names first arose out of a requirement for uniformity as the Internet was quickly growing in size in the 1980s.
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 host with the domain name myhost.example.com.
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.
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).
- RFC 1035, Domain names: implementation and specification
- April N. Marine; Joyce K. Reynolds; Gary Scott Malkin (March 1994). "Questions About the Domain Name System". Answers to Commonly asked "New Internet User" Questions. IETF. sec. 5. RFC 1594. https://tools.ietf.org/html/rfc1594#section-5. Retrieved 29 April 2013. "If you think of the DNS as a tree-structure with each node having its own label, a fully qualified domain name for a specific node would be its label followed by the labels of all the other nodes between it and the root of the tree."
- "The Role of Fully Qualified Domain Names on the Internet". Act Now Domains. Retrieved 20 March 2013.
- Fisher, Tim. "FQDN". About. Retrieved 20 March 2013.
- "Definition of domain names in Simple Mail Transfer Protocol". Tools.ietf.org. 1998-05-21. Retrieved 2014-01-08.