What is a Domain?
A DN is an identification string that defines a realm of administrative autonomy, authority or control within the Internet. They are used in various networking contexts and for application-specific naming and addressing purposes. In general, a DN identifies a network domain, or it represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a server computer hosting a web site, or the web site itself or any other service communicated via the Internet. In 2017, 330.6 million DN had been registered.
They are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a DN. DN are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set is the top-level DN (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level DN (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.
The registration of these DN is usually administered by DN registrars who sell their services to the public.
A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Labels in the Domain Name System are case-insensitive, and may, therefore, be written in any desired capitalization method, but most commonly DN is written in lowercase in technical contexts.