What is meant by FQDN?

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

To ensure completeness, all domain levels such as the root level, top-level domain, subdomains and hostnames are included in the Fully Qualified Domain Name. This makes the domain name system completely unambiguous. Accordingly, any associated IP address of a nameserver can be easily mapped to the DNS (Domain Name System) via the resource records. In contrast to the FQDN, domain names that do not list all domain levels are referred to as PQDN (Partially‑Qualified Domain Name).

Fully Qualified Domain Name - the role 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 one another by a dot. Experts refer to these parts as labels. The root label represents the highest level in the hierarchy. As a rule, it consists only of an empty field. For this reason the FQDN ends with a dot following the top‑level domain. Depending on the application and requirements, this trailing dot can, under certain circumstances, also 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 levels of the hierarchy. These labels are separated from each other by a dot.

To better understand the naming hierarchy of the FQDN, it makes sense to view the structure from right to left. In the hierarchy representation, labels that appear further to the right occupy a higher level than those further to the left.

The highest level is occupied by the root label or null label, which thus represents the 'root' in the Domain Name System. This label consists of an empty string and is therefore indicated only by a dot, which does not need to be entered manually in modern browsers.

The top-level domain occupies the next level down in the hierarchy. Examples include "de", "net" or "com".

The zone for the specified TLD is queried to find the domain at the next lower hierarchical level and thereby to resolve the address of the name server.

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

Restrictions on FQDN assignment

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 permitted as characters. However, a label may not begin with a hyphen. The total number of characters is calculated from letters, numbers and the hyphen.

When is a Fully Qualified Domain Name used?

Anyone seeking reliable identification and reachability of their online presence can use FQDNs to uniquely name an internet resource such as the host. Of course, a valid IPv4 or IPv6 address must be assigned to the fully qualified domain name; otherwise communication on the internet would not be possible. It is the so-called A or AAAA records at a nameserver that enable this. These entries are called resource records and link an IP address to the FQDN. They also authorise the addressing of IP packets with the IP address of the intended destination.

Working with the FQDN

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