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A second-level domain is a specific part of a website, page domain name or URL address that complements a top-level domain. One of the easiest ways to define a second-level domain is that it consists of that portion of the domain name to the left of the ".com" or other similar extension, which is called a top-level domain. Analysis of the top-level and second-level domains helps to analyze a URL or page address.
In a very general sense, a second-level domain is often thought of as the "name" of the domain. The top-level domain, which is an extension such as ".com," is fairly generic. Although it is the controlling address feature, it does not help to distinguish a site from others. The second-level domain often plays this role; for example, in a domain name like "google.com," the word "google," as the second-level domain, is where domain holders put the brand name, project name, organization name or other familiar identifier for users.
In addition to these common second-level domains, there is also the idea of a country code second-level domain (ccSLD). Here, the second-level domain is actually to the right of the decimal delineator; for example, in a domain like "google.co.uk," the country code top-level domain is the "uk" portion, whereas the ccSLD is the ".co."