Domain names are arranged hierarchically. Thus, in the hierarchy of domain names, the top level domain( TLD) is followed by the second level domain( SLD), followed by the third level domain or subdomain.
In the domain name, the various domain levels are arranged in reverse and are always read from back to front. This means that the top level domain is read first, followed by the second level domain, which is separated from the top level domain by a dot. The second level domain is a name, for example of the institution or the company, which is unique under the top level domain. In the domain address http://www.itwissen.de the second level domain is itwissen. Second level domains can be subdivided into subdomains.