What is meant by FQDN?

The abbreviation FQDN stands for Fully Qualified Domain Name. The acronym denotes the complete domain name of an online presence or a host.

To ensure completeness, all domain levels — such as the top-level domain, subdomain and hostname — are included in the Fully Qualified Domain Name. This makes the Domain Name System completely unambiguous. Consequently, any associated IP address of a nameserver can be easily assigned to the DNS (Domain Name System) via the resource records. By contrast, domain names that do not list all domain levels are referred to as PQDN (Partially-Qualified Domain Name).

Fully Qualified Domain Name - the position in the DNS

The FQDN denotes within the DNS (Domain Name System) the complete hierarchy of an online presence and the full domain name for a host. This also includes the root level, the top-level domain, all relevant subdomains and hostnames. The different domain levels are separated from each other by a dot. Experts refer to these sections as labels. The root label represents the highest level in the hierarchy. As a rule it consists only of an empty label. For this reason the FQDN ends with a dot after the top-level domain. Depending on the application and requirements, this dot can, under certain conditions, be omitted.

Syntax and structure

A defined syntax forms the basis for the Fully Qualified Domain Name. The hierarchy of the DNS (Domain Name System) is represented in individual labels that denote the different hierarchy levels. These labels are separated from one another by a dot.

To better understand the name hierarchy in the FQDN, it makes sense to view the structure from right to left. In the hierarchical representation, labels that appear further to the right are higher than those further to the left.

The highest level is held by the root label or null label, which thus represents the 'root' in the Domain Name System. This label consists of an empty label, and is therefore shown only by a dot, which modern browsers no longer require to be entered manually.

The Top-Level Domain sits in the next level down of the hierarchy. Examples include "de", "net" or "com".

The zone of the specified TLD is searched to find the domain at the lower hierarchy level and thus resolve the address of the nameserver.

Once the IP address of the nameserver has been identified, the hostname housed in the lowest label can be contacted and the requested page accessed.

Limitations for assigning an FQDN

When creating a fully qualified domain name, attention must be paid to the number of characters. Each individual label may contain at most 63 characters. The complete fully qualified domain name must also not exceed a total length of 255 characters. Numbers and the hyphen are also permitted as characters. However, a label may not begin with a hyphen. The total number of characters is calculated from the letters, numbers and hyphens.

When is a Fully Qualified Domain Name used?

Anyone aiming for unambiguous identification and accessibility of an internet presence can use an FQDN to uniquely name an internet resource such as a host. Of course, the fully qualified domain name must be associated with a valid IPv4 or IPv6 address. Otherwise communication on the Internet would not be possible. Only so-called A or AAAA records at a name server make this possible. These entries are called resource records and link an IP address to the FQDN. They also enable the addressing of IP packets to the desired destination IP address.

Working with FQDNs

A Fully Qualified Domain Name can be useful for many applications. Among other things, specifying the FQDN may be required to issue an SSL certificate for the server. Contacting a web server via a web browser is done by entering the FQDN in the browser address bar. In this use case, specifying the trailing dot for the root label after the top-level domain is not necessary.