Optionally, another domain level can be subordinated to the domain name. This type of domain is placed before the second-level domain and is called a third-level domain or sub-domain. The function of such a third-level domain is to structure the contents of a website or web store in a meaningful way. Different topics or different language versions of a project can be clearly marked in the web address, while the domain name remains unchanged.
In the domain example www.example.org, the well-known sub-domain www is used. Particularly in the early days of the Internet and the world wide web, this signaled to users that they were dealing with a web application, for example a website with information. Today, however, since it’s no longer necessary to enter the third-level domain in order to visit a website, it’s becoming increasingly rare to come across operators who advertise their website including the “standard” third-level domain, www. More common sub-domains that you encounter much more often nowadays look like the following – staying with the domain example example.org:
- en.example.org
- es.example.org
- it.example.org
- de.example.org
The above shows a typical division into sub-domains for the different language versions of the “example” website. The third-level domain en is used to identify English-language content, while Spanish content can be found under the sub-domain es. The German-language part of the web project runs under the third-level domain de, and Italian users will find content in their native language when visiting the sub-domain it.
In theory, all domains below the top-level domain (TLD) are sub-domains of the preceding one, although in practice they are rarely called sub-domains. The domain name (second-level domain) is a sub-domain of the TLD, the third-level domain is a sub-domain of the second-level domain, and so on. For country-code second-level domains such as “.co” in example.co.uk, the actual domain name (“example”) becomes the third-level domain. A sub-domain of this would then be a fourth-level domain. In principle, the number of domain levels is unlimited. However, to be able to remember an Internet address well, it’s usually limited to three levels.