A domain name is an identification string that defines a group of computers on the internet under the same realm of administrative autonomy, authority, or control. Technically, any name registered in DNS is a domain name, however, the term usually refers to a second and top level domain, such as serverfault.com.

Domains are grouped into a hierarchy from right to left, with levels being delimited by dots. For example, in the domain name chat.stackexchange.com, .com would be the top level domain (TLD), stackexchange would be the second level domain and chat would be the third level domain. (Third level and higher domains are usually referred to as subdomains.)

Domain names serve as human-friendly references to networked devices, which would otherwise be designated by their IP addresses, which are strings of numbers that are generally difficult for humans to remember.

More information on domain names can be found at the Wikipedia page devoted to the topic, here.

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