Overview
A second-level domain is a domain that sits directly below a top-level domain in the hierarchy of the Internet's DNS system. It is the second-to-last part of a domain name and is hosted under a primary domain.
A domain name, also known as a web domain, is composed of letters, numbers, and separators, and is used to identify the electronic location of a computer. A top-level domain is the part of the domain without the "www" prefix. Strictly speaking, domains with "www" are also second-level domains, though many people commonly refer to them as primary domains.
For example, baike300.com is a top-level domain, while www.baike300.com is a second-level domain.
A top-level domain is the highest-level domain partition, with full administrative rights over all subordinate second-level domains — including editing, modification, and storage permissions. In contrast, a second-level domain is an independent branch with its own indexing, cache, PageRank, backlinks, and more. Without proper authorization, a second-level domain cannot access the top-level domain's data.
If a top-level domain is penalized, its associated second-level domains may also be affected. However, if a second-level domain is penalized, the top-level domain is generally unaffected.
If you already own the corresponding top-level domain, many providers will allow you to create second-level domains at no extra cost. For example, VPS Hosting Reviews supports multi-level domains, subdomains, and wildcard domain resolution. However, if the top-level domain for a desired second-level domain is already owned by someone else, you cannot register it.
