Domain name

The term domain name has multiple related meanings:

• A name that identifies a computer or computers on the Internet. These names appear as a component of a Web site's URL, e.g. en.wikipedia.org. This type of domain name is also called a hostname.
• The product that domain name registrars provide to their customers. These names are often called registered domain names.
• Names used for other purposes in the Domain Name System (DNS), for example the special name which follows the @ sign in an email address, or the Top-level domain names like .com, or the names used by the Session Initiation Protocol (VoIP), or DomainKeys.
• They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses".

This article will primarily discuss registered domain names. See the Domain Name System article for technical discussions about general domain names and the hostname article for further information about the most common type of domain name.

The most common types of 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 an intranet), which would then have a different IP address.

By allowing the use of unique alphabetical addresses instead of numeric ones, domain names allow Internet users to more 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 websites), or that one role can be spread among many servers. One IP address can also be assigned to several servers, as used in anycast and hijacked IP space.

Hostnames are restricted to the ASCII letters a through z (case-insensitive), the digits 0 through 9, and the hyphen, with some other restrictions. Registrars restrict the domains to valid hostnames, because they otherwise would be useless. The Internationalized domain name (IDN) system has been developed to bypass the restrictions on character allowances in hostnames, making it easier for users of non-English alphabets to use the Internet. The underscore character is frequently used to ensure that a domain name is not recognized as a hostname, as with the use of SRV records, for example, although some older systems such as NetBIOS did allow it. Due to confusion and other reasons, domain names with underscores in them are sometimes used where hostnames are required.

Domain names are often referred to simply as domains and domain name registrants are frequently referred to as domain owners, although domain names, technically, are leased from a registrar.