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Fully qualified domain name
Domaining Guide
Fully qualified domain name
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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
RFCs —
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 (URL) as it lacks the
protocol name to be used in communication with the host. A URL always starts
with ":", 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.ebay.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.
External links
-
RFC 1035: Domain names: implementation and specification
-
RFC 1123: Requirements for Internet Hosts - application and support
-
RFC 2181: Clarifications to the DNS specification
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Domaining Guide, made by MultiMedia | Websites for sale
This guide is licensed under the GNU
Free Documentation License. It uses material from the Wikipedia.
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