Domain name

The term domain name has multiple related meanings:

A name that is entered into a computer (e.g. as part of a Web site or other URL, or an e-mail address) then looked up in the global Domain Name System (DNS) which informs the computer of the IP address(es) assigned to that name.

The product that registrars provide to their customers.

A name looked up in the DNS for other purposes.

They are sometimes colloquially (and incorrectly) referred to by marketers as "Web addresses".

The authoritative definition is that given in the RFCs that define the DNS.

Domain names are hostnames that provide more memorable names to stand in for numeric IP addresses. They allow for any service to move to a different location in the topology of the Internet (or another internet), which would then have a different IP address.

Each string of letters, digits and hyphens between the dots is called a label in the parlance of the domain name system (DNS). Valid labels are subject to certain rules, which have relaxed over the course of time. The original rules state that labels must start with a letter, and end with a letter or digit; any intervening characters may be letters, digits, or hyphens. Labels must be between 1 and 63 characters long (inclusive). Letters are ASCII A–Z and a–z; domain names are compared case-insensitively. Later it became permissible for labels to commence with a digit (but not for domain names to be entirely numeric), and for labels to contain internal underscores, but support for such domain names is uneven. These are the rules imposed by the way names are looked up ("resolved") by DNS. Some top level domains (see below) impose more rules, such as a longer minimum length, on some labels. Fully qualified domain names (FQDNs) are sometimes written with a final dot.

By making possible the use of unique alphabetical addresses instead of numeric ones, domain names allow Internet users to easily find and communicate with Web sites and other server-based services. The flexibility of the domain name system allows multiple IP addresses to be assigned to a single domain name, or multiple domain names to be assigned to a single IP address. This means that one server may have multiple roles (such as hosting multiple independent Web sites), or one role can be spread among many servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.