DNS

In computer networking, a domain name is a name given to a collection of network devices that belong to a domain which is managed according to some common property of the members or within a common administrative boundary. In particular, the term is used to describe the regions of administrative authority within the Domain name system used for the Internet (cf. DNS zone).
Domain names are used in a variety of contexts for identification, reference, and access to Internet resources. They can appear as components of Web sites' Uniform Resource Locators (URL, 'Web-address'), e.g. www.wikipedia.org, electronic mail (e-mail) addresses after the customary '@' separator from the user's name, or as any other part of a syntax that describes an access method to a device or service in an IP network.
Domain names are created out of a naming space and methodology that was first defined by Paul Mockapetris in IETF publication RFC 882and RFC 883 (1983) and used in the first expansion of the ARPANET, a predecessor of today's Internet. The model prescribed a tree-like structure of named nodes starting from an unnamed root node (cf. DNS root zone) that was only designated by a full stop (period, dot, "."). The complete domain name of each node is the string of names of nodes leading to the root node, each separated by a dot. The sequence is written from left to right with increasing order of scope, e.g., node-d.node-c.node-b.node-a. When the full name path of a node is specified, the domain name is said to be fully-qualified (cf. Fully qualified domain name). This condition is often, particularly in the technical aspects of DNS), indicated explicitly by appending a dot at the end of the name (to indicate the root domain).
The DNS methodology confers a unique name to every resource or service participating in the domain name system. This name is referred to as the domain name of a device or Internet host. However, not all nodes in the tree system denote a specific device, rather they are parent labels of an entire collection of subordinate nodes. Such nodes are the domains of the Internet. They represent the spaces of autonomy that are delegated by a group of service providers, called domain name registrars.
These registrars are authorized and accredited by the Internet Corporation for Assigned Names and Numbers (ICANN), the organization charged with overseeing the name and number systems of the Internet. In addition to IANA, each top-level domain (TLD) is maintained and serviced technically by a sponsoring organization, the TLD Registry. The registry is responsible for maintaining the database of names registered within the TLDs they administer. The registry receives registration information from each domain name registrar authorized to assign names in the corresponding TLD and publishes the information using a special service, the whois protocol.
In this context a domain name is sometimes referred to as a 'product' sold by domain name registrars. However, the rules of assignment specify that no legal ownership is conferred with such transactions, only the right of exclusive use and the authority to the name space. Once assigned, a domain name becomes part of the pool of registered domain names and is no longer available for use by anyone else. Colloquially, marketers incorrectly refer to domain names as "web addresses", however, a web address is actually a fully specified World-Wide Web resource locator, such as http://www.example.org, actually pointing to a web site.
New domain names are usually registered through the registrar for annual terms with a minimum of one year. The maximum length of prepaid registration is often 10 years, but varies depending on the policies of the sponsoring registry of the top-level domain under which registration is sought. Registration periods may be extended, usually at any time, until the end of a grace period after the registration expiration date.
Domain names may be transferred between parties or advertised for 'sale'. This is often called the "domain name aftermarket" (see below). After a domain name registration and the grace period expire, the domain name is either returned to the pool of available names, or receives special treatment by the registrars and could possibly end up in the 'aftermarket'
This article primarily discusses registered domain names. See the Domain Name System article for technical discussions about the domain name system and the hostname article for general discussion of naming aspects of Internet hosts.
The most basic functionality of domain names is to provide symbolic representations, i.e., recognizable names, to mostly 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, in effect changing the IP address. This translation from domain names to IP addresses (and vice versa) is accomplished with the global facilities of Domain Name System (DNS).
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 any other IP-based communications 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 services from 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 networking.
By definition, domain names are restricted to the ASCII letters a through z (case-insensitive), the digits 0 through 9, and the hyphen, with some other restrictions in terms of name length and position of hyphens. Since this does not allow the use of many characters commonly found in non-English languages, and no multi-byte characters necessary for most Asian languages, the Internationalized domain name (IDN) system has been developed and is now in testing stage with a set of top-level domains established for this purpose.
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. To avoid confusion and for 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 name registration with a registrar does not confer any legal ownership of the name, only an exclusive right of use.
Examples
The following example illustrates the difference between a URL (Uniform Resource Locator) and a domain name:
URL: http://www.example.net/index.html
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 208.77.188.166 handles all of the following sites:
example.com
www.example.com
example.net
www.example.net
example.org
www.example.org

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