Fully qualified domain name explained

A fully qualified domain name (or FQDN) is an unambiguous domain name that specifies the node's position in the DNS tree hierarchy absolutely. To distinguish an FQDN from a regular domain name, a trailing period is added. ex: somehost.example.com. An FQDN differs from a regular domain name by its absoluteness; a suffix will not be added.

For example, given a device with a hostname of "myhost" and a domain name of "example.com", the fully qualified domain name is "myhost.example.com.". It therefore uniquely defines the device whilst there might be many hosts in the world called "myhost", there can only be one "myhost.example.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 prefix of a longer domain name such as "myhost.bar.com.au", whereas "myhost.bar.com." is a fully qualified domain name. Technically, the dot comes before the empty label indicating 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 omitted in everyday applications, making such domain references technically ambiguous.

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 RFC RFC 1035, RFC 1123 and RFC 2181. Any binary string can be used as the label of any resource record; a common misconception is that names are limited to a subset of ASCII characters.

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.

Use in URLs and on the Web

A FQDN is not the same as a Uniform Resource Locator as it lacks the protocol name to be used in communication with the host. A URL always starts with "scheme:", and so includes the communication protocol (like "http:", or "ftp:"), and includes information specific to the scheme, such as a directory path, a filename and a TCP port number.

Sometimes FQDNs are specified by the user instead of the full URLs. An example would be typing "www.example.com" into the URL bar of a browser. In this case, the protocol is assumed to be HTTP on TCP port 80; nearly all web browsers use this as the default if not otherwise specified.