What is FQDN (Fully Qualified Domain Name)?
In the context of the internet, when specifying a host (such as a server) for communication, a combination of the host name and the domain name is used. When this combination is written without omission, it is referred to as an FQDN.
For instance, www.f5.com. is an example of an FQDN. The dot at the end signifies the “root domain,” which represents the entirety of the internet structure. The FQDN www.f5.com. indicates the following hierarchy: within the root domain, there exists a top-level domain (com), within which is the second-level domain (f5), and within that domain is the host (www). Although the root domain (final dot) is usually omitted in common usage, making www.f5.com a more typical representation.
While FQDNs are used to specify communication targets, an FQDN does not necessarily map one-to-one with a single host. It's possible for one FQDN to associate with multiple hosts (DNS round-robin) or for multiple FQDNs to point to a single host (virtual host).
Additionally, more websites have been employing hostnames that match their domain names. For example, while www.f5.com is used as an FQDN in the example above, the actual F5 website can also be accessed directly with the FQDN f5.com.