fully qualified domain name#Partially 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,{{Cite IETF|rfc=1035|title=Domain names – Implementation and Specification|doi=10.17487/RFC1035|first1=Paul|last1=Mockapetris}} 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.{{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=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.}} 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
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 root zone, {{code|example}} is nested under {{code|com}}, and finally {{code|somehost}} is nested under {{code|example.com}}.{{Cite IETF |title=Clarifications to the DNS Specification |rfc=2181 |sectionname= |author1=R. Elz |author2=R. Bush |date=July 1997 |publisher=IETF |access-date=27 November 2024 }}
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 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 any top-level domain.{{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}}
The length of each label must be between 1 and 63 octets, 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.{{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 }} It may also be referred to as a partially-qualified domain name, or PQDN.{{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}} 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{{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 }} but are used to reference domains in some TXT records and can appear in resolver configurations, system 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).{{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. }}
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