The term “fully qualified domain name”, FQDN for short, refers to the complete and unique address of an internet presence. It consists of the host name and the domain, and is used to locate specific hosts online and access them using name resolution.
The structure of an FQDN is predefined by the domain name system (DNS). The names of the individual levels in the domain name area are called “labels” and are separated from one another by dots. Each label must consist of 1 to 63 characters and the total FQDN may not exceed 255 characters in total. Only letters, numbers, or dashes can be used. Each label has to have either a letter or a number at the beginning.
The fully qualified domain name consists of three or more labels: The top level domain, the domain names, optional subdomains, and the host name. If a domain name doesn’t specify all of the labels that are required for the FQDN, then this is called a “Partially Qualified Domain Name” (PQDN). This often means that only the host name is given. The generic name or the FQDN and its IP address is the “Fully Qualified Host Name” (FQHN).
In the following, learn exactly how the fully qualified domain name is structured.