A domain is a globally unique and easy-to-remember internet address that serves as a translation for an associated numeric IP address, much like a name for a website or email address.

It consists of several levels: the root label ".", the top-level domain (TLD) such as ".com," ".net," or ".at," the second-level domain, the actual name, and possibly additional subdomains to structure sections of a website.

In the graphic, the FQDN docs.internex.com. is shown in red as an example. A fully qualified domain name, or FQDN, describes the complete domain name.

Using the example docs.internex.com., the following structure results:

Strictly speaking, the FQDN must be preceded by a dot to indicate the root label. In practice, however, the dot is usually omitted, as all domains share the same root label. You can see this, for example, when you look at your browser's address bar.

In the Domain Name System (DNS), where resource records are managed on name servers, the FQDN must be preceded by a dot.

Top-level domains, the uppermost level behind the root label, can be divided into several categories:

gTLDs (generic TLDs) can be further divided into uTLDs (unsponsored TLDs) and sTLDs (sponsored TLDs).

ccTLDs (country-code TLDs) are assigned to countries or territories. Country codes are assigned according to ISO standards. The administration of ccTLDs is largely handled by the respective countries themselves, resulting in different legal regulations and standards.

iTLDs (Infrastructure TLDs) are used to manage and operate the basic infrastructure of the Internet. These TLDs are not available to the public.

A second-level domain is usually the actual, unique part of a domain. In most cases, this can be freely chosen during registration.

The FQDN docs.internex.com., therefore, represents "internex," our company name, as the second-level domain.

An exception to this is so-called ccSLDs, such as ".co.at.", where the ".co" represents the SLD and the actual, freely selectable part is a third-level domain.

Third-level domains are usually used to subdivide a domain to provide structured access to different services.

So, going back to our example of docs.internex.com., "docs" would be the subdomain that makes our knowledge base available online. If you switch to the other subdomain, terminal.internex.com., for example, you reach our customer portal.

In principle, any number of subdomains or levels can be created. In practice, it is not recommended to use more than three or four nested subdomains. FQDNs that are too long can impair services such as certificate issuance or mailing.

The total length of the FQDN must not exceed 255 characters (including subdivisions).

Each individual subdomain, including the SLD, must contain a maximum of 63 characters.