The Domain NameSystem(DNS) is subject to a strict hierarchical structure and consists of the Top Level Domain ( TLD), the Second Level Domain( SLD) and the Third Level Domain, also known as a subdomain.
In the notation, the individual domain levels are arranged from the back to the front: i.e. the TLD last, the SLD before it, separated by a dot, and the third level domain before it, again separated by a dot.
The third level domain offers large companies or strongly structured websites the possibility of a logical structure and the separation of workplaces, departments or product offers. For example, a sales organization with the TLD "com" and the SLD "abc" could offer different product groups under different third level domains.
For example, it could use the third level domain "foto" for photo items, "phono" for phono items, and "books" for books. The different domain names would then look like this: www.foto.abc.com, www.phono.abc.com and www.books.abc.com.