Afully qualified domain name (FQDN), sometimes also called anabsolute domain name,[1] is a domain name that specifies its exact location in the tree hierarchy of theDomain Name System (DNS). It specifies all domain levels, including thetop-level domain and the root zone.[2] A fully qualified domain name is distinguished by its unambiguous DNS zone location in the hierarchy of DNS labels: it can be interpreted only in one way.
A fully qualified domain name is conventionally written as a list of domain labels separated using the full stop "." character (dot orperiod). The top of the hierarchy in an FQDN begins with the rightmost label. For instance, in the FQDNsomehost.example.com,com is a label directly under the root zone,example is nested undercom, and finallysomehost is nested underexample.com.[3]
The topmost layer of every domain name is the DNS root zone, which is expressed as an empty label and can be represented in an FQDN with a trailing dot, such assomehost.example.com.. A trailing dot is generally implied and often omitted by most applications. Trailing dots are required by the standard format for DNSzone files, as well as to disambiguate cases where an FQDN does not contain any other label separators, such as the FQDNs for the root zone itself and anytop-level domain.[4]
The length of each label must be between 1 and 63octets, and the full domain name is limited to 255 octets, full stops included.[3]
Arelative domain name is a domain name which does not include all labels.[5] It may also be referred to as a partially-qualified domain name, or PQDN.[6]Hostnames can be used as relative domain names.
Dot-separated fully qualified domain names are the primarily used form for human-readable representations of a domain name. Dot-separated domain names are not used in the internal representation of labels in a DNS message[7] but are used to reference domains in someTXT records and can appear inresolver configurations, systemhosts files, andURLs.
Web addresses typically use FQDNs to represent the host, as it ensures the address will be interpreted identically on any network. Relative hostnames are allowed by some protocols, includingHTTP, but disallowed by others, such as theSimple Mail Transfer Protocol (SMTP).[8]
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.