FQDN
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A fully qualified domain name (or FQDN) is the human-readable name corresponding to the TCP/IP address of a network interface, as found on a computer, router or other networked device. It includes both its host name and its domain name.
For example, given a device with a hostname of "myhost" and a domain name of "bar.com", the fully qualified domain name is "myhost.bar.com.". It therefore uniquely defines the device — whilst there might be many hosts in the world called "myhost", there can only be one "myhost.bar.com.".
Notice that there is a dot at the very end of the domain name, i.e. it ends ".com." and not ".com" — this indicates that the name is an FQDN. For example "myhost.bar.com" could be ambiguous, because it could be the suffix of a longer domain name such as "myhost.bar.com.gov", whereas "myhost.bar.com." is a fully qualified domain name. Technically, the dot is the root of the Domain Name System hierarchy, and so an FQDN is sometimes called a rooted domain name. In practice, the dot is almost always left out in web pages and other documents making the domain name ambiguous, at least in theory.
The maximum permitted length of an FQDN is 255 bytes, with an additional restriction to 63 bytes for each label within the domain name. The syntax of domain names is discussed in various RFCs — RFC 1035, RFC 1123 and RFC 2181. Labels in FQDNs are restricted to a limited character set, consisting of the ASCII letters A to Z, digits, and the "-" character, and are not case-sensitive.
Internationalized domain names expand the character repertoire of domain names to include non-ASCII characters, by encoding Unicode characters into byte strings within the normal FQDN character set. As a result, the character length limits of internationalized domain names are content-dependent.
A FQDN is not the same as a Universal Resource Locator (URL) as it lacks the TCP/IP protocol name to be used in communication with the host. A URL always starts with "<protocol>://", and so includes the communication protocol (like "http://", or "ftp://"), and can include a directory path, a filename and a TCP port number too.
Sometimes FQDNs are specified instead of the full URLs on websites etc., in which case the protocol is assumed to be HTTP on TCP port 80; and web browsers use this as the default if it is not otherwise specified.
External links
- RFC 1035: Domain names: implementation and specification
- RFC 1123: Requirements for Internet Hosts - application and support
- RFC 2181: Clarifications to the DNS specificationde:Fully Qualified Domain Name