Editing Fully qualified domain name

{{Short description|Type of Internet domain name}}
{{Use dmy dates|date=November 2024}}

A '''fully qualified domain name''' ('''FQDN'''), sometimes also called an '''absolute domain name''',<ref name="RFC1035">{{Cite IETF|rfc=1035|title=Domain names – Implementation and Specification|doi=10.17487/RFC1035|first1=Paul|last1=Mockapetris}}</ref> 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 [[DNS root zone|root zone]].<ref>{{cite IETF |title=Answers to Commonly asked "New Internet User" Questions |rfc=1594 |sectionname=Questions About the Domain Name System |section=5 |author1=April N. Marine |author2=Joyce K. Reynolds |author3=Gary Scott Malkin |date=March 1994 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=29 April 2013 |quote=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.}}</ref> 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.

==Definition==
[[File:DNS schema.svg|The hierarchy of labels in a fully qualified domain name.|thumb|350px|alt=|right]] 
A fully qualified domain name is conventionally written as a list of domain labels separated using the [[full stop]] "{{code|.}}" character (''dot'' or ''period''). The top of the hierarchy in an FQDN begins with the rightmost label. For instance, in the FQDN {{code|somehost.example.com}}, {{code|com}} is a label directly under the [[DNS root zone|root zone]], {{code|example}} is nested under {{code|com}}, and finally {{code|somehost}} is nested under {{code|example.com}}.<ref name="rfc2181">{{Cite IETF |title=Clarifications to the DNS Specification |rfc=2181 |sectionname= |author1=R. Elz |author2=R. Bush |date=July 1997 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=27 November 2024 }}</ref>

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 as {{code|somehost.example.com.}}. A trailing dot is generally implied and often omitted by most applications. Trailing dots are required by the standard format for DNS [[zone file]]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 [[top-level domain]].<ref>{{cite web|last=Fisher|first=Tim|title=FQDN|url=http://pcsupport.about.com/od/termsf/g/fqdn-fully-qualified-domain-name.htm|publisher=About.com|access-date=20 March 2013|archive-date=3 April 2013|archive-url=https://web.archive.org/web/20130403142819/http://pcsupport.about.com/od/termsf/g/fqdn-fully-qualified-domain-name.htm|url-status=dead}}</ref>

The length of each label must be between 1 and 63 [[Octet (computing)|octets]], and the full domain name is limited to 255 octets, full stops included.<ref name="rfc2181" />

==Relative domain names==
A '''relative domain name''' is a domain name which does not include all labels.<ref>{{Cite web|url = https://tools.ietf.org/html/rfc1535|title = A Security Problem and Proposed Correction With Widely Deployed DNS Software| date=October 1993 |access-date = 1 October 2020|archive-date = 17 October 2020|archive-url = https://web.archive.org/web/20201017033617/https://tools.ietf.org/html/rfc1535|url-status = live | last1=Gavron | first1=Ehud }}</ref> It may also be referred to as a partially-qualified domain name, or PQDN.<ref>{{Cite web|url = http://www.omnisecu.com/tcpip/fully-qualified-domain-name-fqdn-and-partially-qualified-domain-name-pqdn.php|title = Fully Qualified Domain Name (FQDN) and Partially Qualified Domain Name (PQDN)|access-date = 23 March 2015|archive-date = 16 March 2015|archive-url = https://web.archive.org/web/20150316151130/http://www.omnisecu.com/tcpip/fully-qualified-domain-name-fqdn-and-partially-qualified-domain-name-pqdn.php|url-status = live}}</ref> [[Hostname]]s 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<ref>{{cite web |title=Wireshark Q&A |url=https://osqa-ask.wireshark.org/questions/50806/help-understanding-dns-packet-data/ |website=osqa-ask.wireshark.org |access-date=13 April 2021 |archive-date=13 April 2021 |archive-url=https://web.archive.org/web/20210413010527/https://osqa-ask.wireshark.org/questions/50806/help-understanding-dns-packet-data/ |url-status=live }}</ref> but are used to reference domains in some [[TXT record|TXT records]] and can appear in [[resolver (DNS)|resolver]] configurations, system [[hosts (file)|hosts files]], and [[URLs]].

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]], but disallowed by others, such as the [[Simple Mail Transfer Protocol]] (SMTP).<ref>{{cite web |url=http://tools.ietf.org/html/rfc5321#section-2.3.5 |title=Definition of domain names in Simple Mail Transfer Protocol |publisher=Tools.ietf.org |date=21 May 1998 |access-date=8 January 2014 |archive-date=30 December 2013 |archive-url=https://web.archive.org/web/20131230053037/http://tools.ietf.org/html/rfc5321#section-2.3.5 |url-status=live |last1=Klensin |first1=John C. }}</ref>

==References==
{{Reflist}}

==External links==
* {{IETF RFC|1123}}: Requirements for Internet Hosts – application and support
* {{IETF RFC|1535}}: A Security Problem and Proposed Correction With Widely Deployed DNS Software
* {{IETF RFC|2181}}: Clarifications to the DNS specification
* {{IETF RFC|4703}}: Resolution of Fully Qualified Domain Name (FQDN) Conflicts among Dynamic Host Configuration Protocol (DHCP) Clients

[[Category:Domain Name System]]

[[de:Domain (Internet)#Fully Qualified Domain Name (FQDN)]]