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A domain name is an identification label that defines a realm of administrative autonomy, authority, or control in the Internet, based on the Domain Name System (DNS).

Domain names are used in various networking contexts and application-specific naming and addressing purposes. They are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, net and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users that wish to connect local area networks to the Internet, run web sites, or create other publicly accessible Internet resources. The registration of these domain names is usually administered by domain name registrars who sell their services to the public.

Individual Internet host computers use domain names as host identifiers, or hostnames. Hostnames are the leaf labels in the domain name system usually without further subordinate domain name space. Hostnames appear as a component in Uniform Resource Locators (URLs) for Internet resources such as web sites (e.g., en.wikipedia.org).

Domain names are also used as simple identification labels to indicate ownership or control of a resource. Such examples are the realm identifiers used in the Session Initiation Protocol (SIP), the DomainKeys used to verify DNS domains in e-mail systems, and in many other Uniform Resource Identifiers (URIs).

An important purpose of domain names is to provide easily recognizable and memorizable names to numerically addressed Internet resources. This abstraction allows any resource (e.g., website) to be moved to a different physical location in the address topology of the network, globally or locally in an intranet. Such a move usually requires changing the IP address of a resource and the corresponding translation of this IP address to and from its domain name.

Domain names are often referred to simply as domains and domain name registrants are frequently referred to as domain owners, although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use.

This article primarily discusses the group of domain names that are offered by domain name registrars for registration by the public. The Domain Name System article discusses the technical facilities and infrastructure of the domain name space and the hostname article deals with specific information about the use of domain names as identifiers of network hosts.

Top-level domains

The top-level domains (TLDs) are the highest level of domain names of the Internet. They form the DNS root zone of the hierarchical Domain Name System. Every domain name ends in a top-level or first-level domain label.

When the Domain Name System was created in the 1980s, the domain name space was divided into two main groups of domains. The country code top-level domains (ccTLD) were primarily based on the two-character territory codes of ISO-3166 country abbreviations. In addition, a group of seven generic top-level domains (gTLD) was implemented which represented a set of categories of names and multi-organizations. These were the domains GOV, EDU, COM, MIL, ORG, NET, and INT.

During the growth of the Internet, it became desirable to create additional generic top-level domains. As of June 2009, there are 20 generic top-level domains and 248 country code top-level domains. In addition, the ARPA domain serves technical purposes in the infrastructure of the Domain Name System.

During the 32nd International Public ICANN Meeting in Paris in 2008, ICANN started a new process of TLD naming policy to take a "significant step forward on the introduction of new generic top-level domains." This program envisions the availability of many new or already proposed domains, as well a new application and implementation process. Observers believed that the new rules could result in hundreds of new top-level domain to be registered.

Second-level and lower level domains

Below the top-level domains in the domain name hierarchy are the second-level domain (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain en.wikipedia.org, wikipedia is the second-level domain.

Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An example of an operational domain name with four levels of domain labels is www.sos.state.oh.us. The www preceding the domains is the host name of the World-Wide Web server. Each label is separated by a full stop (dot). 'sos' is said to be a sub-domain of 'state.oh.us', and 'state' a sub-domain of 'oh.us', etc. In general, subdomains are domains subordinate to their parent domain. An example of very deep levels of subdomain ordering are the IPv6 reverse resolution DNS zones, e.g., 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which is the reverse DNS resolution domain name for the IP address of a loopback interface, or the localhost name.

Second-level (or lower-level, depending on the established parent hierarchy) domain names are often created based on the name of a company (e.g., microsoft.com), product or service (e.g., gmail.com). Below these levels, the next domain name component has been used to designate a particular host server. Therefore, ftp.wikipedia.org might be an FTP server, www.wikipedia.org would be a World Wide Web server, and mail.wikipedia.org could be an email server, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. load balancing) or even identical addresses (cf. anycast) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in Web hosting service centers, where service providers host the websites of many organizations on just a few servers.

Allowed character set

Domain name registrations have traditionally only been allowed for names that consist only of letters, digits and the hyphen (-) from the ASCII character set, as in hostnames. This character set excludes characters commonly found in non-English languages, and does not allow multi-byte characters necessary for most Asian languages. Internationalized domain names (IDNs) permit such characters and many registries allow such names as second-level domains. As of November 16, 2009, ICANN is accepting applications for IDN country code top-level domains (ccTLDs) that represent country-specific names in the language-native writing system consisting of non-Latin symbols. Such top-level domains are expected to be installed in the DNS root for successful applications in 2010.

Internationalized domain names are represented in the Domain Name System in the original DNS character set, uniquely transcribed using Punycode on a per-label basis. In applications, the IDN domain names are represented in multi-byte unicode, suitable for display of the native script to users. However, applications must be appropriately designed to use the IDN domain names. The infrastructure of the Internet does not require any modifications.

The underscore character is permitted in names used in the Domain Name System, and is frequently used to ensure that a domain name is not recognized as a hostname, as in the use of DNS server records (SRV), for example. Other naming systems often used in conjunction with DNS, such as NetBIOS, allow it.

The hierarchical DNS labels or components of domain names are separated in a fully qualified name by the full stop (dot, .).

Official assignment

The Internet Corporation for Assigned Names and Numbers (ICANN) has overall responsibility for managing the DNS. It administers the root domain, delegating control over each TLD to a domain name registry. For ccTLDs, the domain registry is typically installed by the government of that country. ICANN has a consultation role in these domain registries but cannot regulate the terms and conditions of how domain names are delegated in each of the country-level domain registries. On the other hand, the 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 in analogy to real estate in that (1) domain names are foundations 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, and many other criteria.

A few companies have offered low-cost, below-cost or even cost-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 give away or sell infinite number of subdomains under 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.

Resale of domain names

The business of resale of previously registered domain names is known as the domain aftermarket. Various factors influence the perceived value or market value of a domain name. Annually tens of millions of dollars change hands in connection with the resale of domains. Large numbers of registered domain names lapse and are deleted each year. On average, more than 25,000 domain names drop (are deleted) every day.

Domain aftermarket prices and trends

Domain name sales occurring in the aftermarket are frequently submitted to the DN journal. The sales are listed weekly and include the top aftermarket resellers which include but are not limited to Sedo, Traffic (auctions), Afternic, NameJet, Moniker and private sales.

To date, and according to Guinness World Records and MSNBC, the most expensive domain name sales on record as of 2004 were:

* Business.com for $7.5 million in December 1999
* AsSeenOnTv.com for $5.1 million in January 2000
* Altavista.com for $3.3 million in August 1998
* Wine.com for $2.9 million in September 1999
* CreditCards.com for $2.75 million in July 2004
* Autos.com for $2.2 million in December 1999

Domain name confusion

Intercapping is often used to emphasize the meaning of a domain name. However, DNS names are case-insensitive, and some names may be misinterpreted in certain uses of capitalization. For example: Who Represents, a database of artists and agents, chose whorepresents.com, which can be misread as whore presents. Similarly, a therapists' network is named therapistfinder.com. In such situations, the proper meaning may be clarified by use of hyphens in the domain name. For instance, Experts Exchange, the programmers' site, for a long time used expertsexchange.com, but ultimately changed the name to experts-exchange.com.

Leo Stoller threatened to sue the owners of StealThisEmail.com on the basis that, when read as stealthisemail.com, it infringed on claimed (but invalid) trademark rights to the word "stealth".

Use in web site hosting

A domain name is a component of a Uniform Resource Locator (URL) used to access web sites, for example:

URL: http://www.example.net/index.html
Domain name: www.example.net
Second-level domain name: example.net

A domain name may point to multiple IP addresses to provide server redundancy for the services delivered. This is used for large, popular web sites. More commonly, however, one server at a given IP address may also host multiple web sites in different domains. Such address overloading is possible through a feature in the HTTP version 1.1 protocol (but not in HTTP 1.0) which requires that a request identifies the domain name being referenced. This enables virtual web hosting commonly used by large web hosting services to conserve IP address space.

For example, the web server at IP address 208.77.188.166 handles all HTTP page requests to the following domain names:

example.com
www.example.com
example.net
www.example.net
example.org
www.example.org