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.
The following example illustrates the difference between a URL (Uniform Resource Locator) and a domain name:
Domain name: www.example.net
Registered domain name: example.net
As a general rule, the IP address and the server name are interchangeable. For most Internet services, the server will not have any way to know which was used. However, the explosion of interest in the Web means that there are far more Web sites than servers. To accommodate this, the hypertext transfer protocol (HTTP) specifies that the client tells the server which name is being used. This way, one server with one IP address can provide different sites for different domain names. This feature goes under the name virtual hosting and is commonly used by Web hosts.
For example, as referenced in RFC 2606 (Reserved Top Level DNS Names), the server at IP address 18.104.22.168 handles all of the following sites:
When a request is made, the data corresponding to the hostname requested is served to the user.
ICANN (Internet Corporation for Assigned Names and Numbers) has overall responsibility for managing the DNS. It controls the root domain, delegating control over each TLD to a domain name registry. For ccTLDs, the domain registry is typically controlled by the government of that country. ICANN has a consultation role in these domain registries but is in no position to regulate the terms and conditions of how a domain name is allocated or who allocates it in each of these country-level domain registries. On the other hand, generic top-level domains (gTLDs) are governed directly under ICANN, which means all terms and conditions are defined by ICANN with the cooperation of each gTLD registry.
Domain names are often seen as being similar to real estate in that (1) domain names are virtual properties on which a website (like a house or commercial building) can be built and (2) the highest quality domain names, like sought-after real estate, tend to carry significant value, usually due to their online brand-building potential, use in advertising, search engine optimization, etc.
A few companies have offered low-cost, below-cost or even free domain registrations, with a variety of models adopted to recoup the costs to the provider. These usually require that domains be hosted on their website within a framework or portal that includes advertising wrapped around the domain holder's content, revenue from which allows the provider to recoup the costs. Domain registrations were free of charge when the DNS was new. A domain holder (often referred to as a domain owner) can generally give away or sell infinite subdomains on their domain name. For example, the owner of example.edu could provide subdomains such as foo.example.edu and foo.bar.example.edu to interested parties.
Uses and abuses
As domain names became attractive to marketers—rather than just the technical audience for which they were originally intended—they began to be used in manners that in many cases did not fit in their intended structure. As originally planned, the structure of domain names followed a strict hierarchy in which the TLD indicated the type of organization (commercial, governmental, etc.), and addresses would be nested down to third, fourth, or further levels to express complex structures, where, for instance, branches, departments and subsidiaries of a parent organization would have addresses that were subdomains of the parent domain. Also, hostnames were originally intended to correspond to actual physical machines on the network, generally with only one name per machine.
However, once the World Wide Web became popular, site operators frequently wished to have memorable addresses, regardless of whether they fit properly into the structure; thus, because the .com domain was the most popular and memorable, even noncommercial sites began to obtain domains in that gTLD, and sites of all sorts wished to have second-level domain registrations even if they were parts of a larger entity where a subdomain would have been logical (e.g., abcnews.com instead of news.abc.com). A website found at ''http://www.example.org'' will often be advertised without the http:// and, in most cases, can be reached by just entering example.org into a web browser. In the case of a .com, the website can sometimes be reached by just entering example (depending on browser versions and configuration settings, which vary in how they interpret incomplete addresses).
The popularity of domain names also led to uses which were regarded as abusive by established companies with trademark rights; this has become known as cybersquatting, in which a person registers a domain name that resembles a trademark in order to profit from visitors looking for that address. To combat this, various laws and policies were enacted to allow abusive registrations to be forcibly transferred, but these were sometimes themselves abused by overzealous companies committing reverse domain hijacking against domain users who had legitimate grounds to hold their names. Such legitimate uses could include the use of generic words that are contained within a trademark, but used in a particular context within the trademark, or their use in the context of fan or protest sites with free speech rights of their own.
Laws that specifically address domain name conflicts include the Anticybersquatting Consumer Protection Act in the United States and the Trademarks Act of 1999 in India. Alternatively, domain registrants are bound by contract under the UDRP to comply with mandatory arbitration proceedings should someone challenge their ownership of a domain name.