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Fully qualified domain name explained

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There is much more behind a domain than one imagines. They conceal a unique technical cosmos. Their resolution in the domain name system (DNS) is a complex process, as each computer, each domain and each IP address must be unambiguously identifiable. How does this work? This is where the so-called fully qualified domain name (short FQDN) comes into play.

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Carina Prüll

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The term fully qualified domain name refers to a domain name that fully specifies the unique and exact location of a domain name within the DNS. The DNS provides the syntax. An FQDN is made up of individual labels that are each delimited by a dot and represent the different levels of the FQDN.

[hostname].[domain].[tld]   -> [www].[internetx].[com]

The FQDN is relevant whenever a connection to the host is established. The DNS server resolves the hostname to its IP address by querying the DNS database. So, an FQDN shows the absolute path to a host in a complete form that is legible to users, and the DNS translates it into the host's IP address.

There are a few rules for these so-called “absolute addresses”:

  • a maximum length of 255 octets (RFC 1035), which in practice means 253 characters in written form, with up to 63 characters per label
  • only numbers, letters and hyphens are allowed
  • a number or letter must take the first position of each label

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How is a fully qualified domain name structured?

The hierarchy levels of an FQDN are read from right to left. The root label, also known as the null label, takes the highest hierarchy level and is therefore, as the name suggests, the root of the DNS system. It consists of an empty string and is indicated with a simple full stop. In everyday use this trailing dot is usually left out because the root is implied, so users do not need to enter it.

The further right it is positioned, the higher the level held in the DNS. So the domain holds a higher level than a subdomain.

Root domain

While most people are aware of the reading direction for domain names, the root domain usually remains hidden. It is the basis of the DNS system and is also called null or root label. It is made up of an empty zero-length string consisting of a single octet of zeros. In an FQDN, which strictly follows syntax, it is simply indicated by a dot.

Technically, a top-level domain is therefore in actual fact also a subdomain of the root domain. Because the root is implied, the trailing dot is usually omitted when domain names are written or typed.

Top-level domain

In our example, the top-level domain is “.com”. Under this domain extension, the actual desired domain name can be selected in accordance with the guidelines of the respective registry.

Second-level domain

The second-level domain constitutes the address referred to as “domain”. Some registries also offer a predetermined second level, so the selectable name is located on the third level, as with .co.uk or .org.uk. Since 10 June 2014, domains can also be registered directly under .uk. The same applies to the Australian top-level domain: since 24 March 2022, second-level domains can be registered directly under .au, alongside third-level registrations such as .com.au.

Hostname

As mentioned before, FQDNs are read from right to left. Between the second-level domain and the hostname, further subdomains may be included, numbered consecutively as third-level domain, fourth-level domain and so on. Farthest left, the hostname follows at the end. In our examplehttps://www.internetx.com/en/domains-and-dns-services/, the hostname is “www.” So, while a host name is always a subdomain, a subdomain is not necessarily already the hostname.

What is the difference between FQDN, FQHN and PQDN?

Via a fully qualified hostname (FQHN), a query reaches a unique and unambiguously identifiable computer. Its address is indicated through an IP or an FQDN.

An example
The computer with the name “mycomputer” can either be reached via the FQDN mycomputer.internetx.com or via an IP address, such as the fictive example 85.236.36.86.

By contrast, the partially qualified domain name (PQDN) is a relative name for a domain. It does not provide the complete path to the top-level domain. The name “mycomputer” is the PQDN in our example mycomputer.internetx.com.

A tip for further reading
3 domain strategies for your multilingual website architecture in our blog article “Domains for Multilingual Websites | Strategy & Tips”.

Looking for interesting domains to add to your portfolio? Don’t have an AutoDNS account yet? You should get one today.

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