A Fully Qualified Domain Name (or FQDN) is the human-readable TCP/IP 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 a Fully Qualified Domain Name. 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 heirarchy, and so an FQDN is sometimes called a rooted domain name. In practice, the dot is frequently left out in web pages and other documents making the domain name ambiguous, at least in theory.
The maximum permitted length of a Fully Qualified Domain Name is 255 characters. The syntax of domain names is discussed in various RFCs — RFC1035, RFC1123 and RFC2181.
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 "://", 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 Fully Qualified Domain Names are specified instead of the full URLs on websites etc., in which case the protocol is assumed to be http; and web browserA web browser is a software package that enables a user to display and interact with documents hosted by web servers. Popular browsers include Microsoft Internet Explorer and Mozilla Firefox. A browser is the most commonly used kind of user agent. The lars use this as the default if it is not otherwise specified.
