A fully qualified domain name (FQDN), sometimes also referred to as an ''absolute domain name'', is a

domain name A domain name is a string that identifies a realm of administrative autonomy, authority or control within the Internet. Domain names are often used to identify services provided through the Internet, such as websites, email services and more. As ...

that specifies its exact location in the tree hierarchy of the

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...

(DNS). It specifies all domain levels, including the

top-level domain A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the root zone of the name space. For all domains in ...

and the root zone. A fully qualified domain name is distinguished by its lack of ambiguity in terms of DNS zone location in the hierarchy of DNS labels: it can be interpreted only in one way.

Definition

A fully qualified domain name is conventionally written as a list of domain labels separated using the

full stop The full stop (Commonwealth English), period (North American English), or full point , is a punctuation mark. It is used for several purposes, most often to mark the end of a declarative sentence (as distinguished from a question or exclamatio ...

“” character (''dot'' or ''period''). The top of the hierarchy in an FQDN begins with the rightmost label. For instance, in the FQDN , is a label directly under the root zone, is nested under , and finally is nested under . The topmost layer of every domain name is the

DNS root zone The DNS root zone is the top-level DNS zone in the hierarchical namespace of the Domain Name System (DNS) of the Internet. Before October 1, 2016, the root zone had been overseen by the Internet Corporation for Assigned Names and Numbers (ICAN ...

, which is expressed as an empty label and can be represented in an FQDN with a trailing dot, such as . A trailing dot is generally implied and often omitted by most applications. Trailing dots are required by the standard format for DNS

zone file A Domain Name System (DNS) zone file is a text file that describes a DNS zone. A DNS zone is a subset, often a single domain, of the hierarchical domain name structure of the DNS. The zone file contains mappings between domain names and IP add ...

s, 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 any

. The length of each label must be between 1 and 63

octets Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compo ...

, and the full domain name is limited to 255 octets, full stops included.

Relative domain names

A relative domain name is a domain name which does not include all labels. It may also be referred to as a partially-qualified domain name, or PQDN. Hostnames can be used as relative domain names.

Usage

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 but are used to reference domains in some TXT records and can appear in resolver configurations, system hosts files, and

URLs A Uniform Resource Locator (URL), colloquially termed as a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...

. 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, including

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

, but disallowed by others, such as the Simple Mail Transfer Protocol (SMTP).

References

External links

* : Requirements for Internet Hosts – application and support * : A Security Problem and Proposed Correction With Widely Deployed DNS Software * : Clarifications to the DNS specification * {{IETF RFC, 4703: Resolution of Fully Qualified Domain Name (FQDN) Conflicts among Dynamic Host Configuration Protocol (DHCP) Clients Domain Name System de:Domain (Internet)#Fully Qualified Domain Name (FQDN)