FQDN
Encyclopedia
A fully qualified domain name (FQDN), sometimes also referred 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 domain. A fully qualified domain name is distinguished by its unambiguity; it can only be interpreted one way.
For example, 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 myhost.example.com.
In the Domain Name System, and most notably, in DNS zone file
s, a fully qualified domain name is specified with a trailing dot
. For example,
specifies an absolute domain name that ends with an empty top level domain label.
The DNS root domain is unnamed, which is expressed by an empty label, resulting in a domain name ending with the dot separator. However, many DNS resolvers process a domain name that contains a dot in any position as being fully qualifiedNote: On Unix-like systems, this is controlled by the
configuration file, specifying the number of dots (default 1) recognized to imply a FQDN. There are some security issues in connection with this interpretation as discussed in RFC 1535. 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 browser
s 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
(e-mail).
There are 13 authoritative root nameserver
s which contain the DNS
records for root name lookups. Each name server knows the IP addresses of the name servers of first or "top level domains" (TLDs). The "com." and "uk." domains are TLDs.
Just like the root domain, most TLDs do not resolve to an IP address, but usually have three or more distinct name servers which answer queries for the TLD. (e.g. There is no server known by the FQDN "net." nor "uk." but there are 13 name servers listed for "net." and 11 name servers "uk.")
An example of a TLD which resolves is "uz.", meaning that the .uz
domain is an example of the shortest resolving FQDN with a URL ofhttp://uz/ for web access and is notable because no dot appears in the URL. Due to the scarcity of domains without a dot, not all browsers will permit this to work.
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
that specifies its exact location in the tree hierarchy of the Domain Name System
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS). It specifies all domain levels, including the top-level domain
Top-level domain
A top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...
and the root domain. A fully qualified domain name is distinguished by its unambiguity; it can only be interpreted one way.
For example, 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 myhost.example.com.
In the Domain Name System, and most notably, in DNS zone file
Zone file
A Domain Name System 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 addresses and other resources, organized in the form of text...
s, a fully qualified domain name is specified with a trailing dot
Full stop
A full stop is the punctuation mark commonly placed at the end of sentences. In American English, the term used for this punctuation is period. In the 21st century, it is often also called a dot by young people...
. For example,
- somehost.example.com.
specifies an absolute domain name that ends with an empty top level domain label.
The DNS root domain is unnamed, which is expressed by an empty label, resulting in a domain name ending with the dot separator. However, many DNS resolvers process a domain name that contains a dot in any position as being fully qualifiedNote: On Unix-like systems, this is controlled by the
ndots option in the resolv.confResolv.conf
resolv.conf is the name of a computer file used in various operating systems to configure the Domain Name System resolver library. The file is a plain-text file usually created by the network administrator or by applications that manage the configuration tasks of the system...
configuration file, specifying the number of dots (default 1) recognized to imply a FQDN. There are some security issues in connection with this interpretation as discussed in RFC 1535. 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 browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
s try to resolve the domain name part of a Uniform Resource Locator
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
(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
Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
(e-mail).
The root of the tree and DNS
As a special case to the FQDN, a single dot should represent the root of the directory tree, which in turn would mean that a hypothetical computer system would be called the root server. There is no such thing as a root server on the global Internet, however, since there is no A record for the "." domain.There are 13 authoritative root nameserver
Root nameserver
A root name server is a name server for the Domain Name System's root zone. It directly answers requests for records in the root zone and answers other requests returning a list of the designated authoritative name servers for the appropriate top-level domain...
s which contain the DNS
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
records for root name lookups. Each name server knows the IP addresses of the name servers of first or "top level domains" (TLDs). The "com." and "uk." domains are TLDs.
Just like the root domain, most TLDs do not resolve to an IP address, but usually have three or more distinct name servers which answer queries for the TLD. (e.g. There is no server known by the FQDN "net." nor "uk." but there are 13 name servers listed for "net." and 11 name servers "uk.")
An example of a TLD which resolves is "uz.", meaning that the .uz
.uz
.uz is the Internet country code top-level domain for Uzbekistan. Registry services were formerly operated by Euracom GmBH, but were later redelegated to UZINFOCOM...
domain is an example of the shortest resolving FQDN with a URL of
External links
- RFC 1123: Requirements for Internet Hosts - application and support
- RFC 1535: A Security Problem and Proposed Correction With Widely Deployed DNS Software
- RFC 2181: Clarifications to the DNS specification
- RFC 2826: IAB Technical Comment on the Unique DNS Root
- http://uz/, a URL with no dot because it is a top-level domain