About Domains

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. xx.topcenterzero.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 (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".

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.

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, (Reserved Top Level DNS Names), the server at IP address 208.77.188.166 (Reserved Top Level DNS Names) handles all of the following sites:

example.com - www.example.com

example.net - www.example.net

example.org - www.example.org

When a request is made, the data corresponding to the hostname requested is served to the user.

Top-level domains

Every domain name ends in a top-level domain (TLD) name, which is always either one of a small list of generic names (three or more characters), or a two-character territory code based on ISO-3166 (there are few exceptions and new codes are integrated case by case). Top-level domains are sometimes also called first-level domains.

The generic top-level domain> (gTLD) extensions are:

Other-level domains

In addition to the top-level, or root, domains, there are 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 xx.anycenter.org, anycenter is the second-level domain.

Next are third-level domains, which are shown immediately to the left of a second-level domain. In the xx.anycenter.org example, xx is a third-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An possible working domain with five levels could be www.sos.please.save.us. Each level is separated by a dot, or period symbol.

Domains of third or higher levels are also known as subdomains, though this term technically applies to a domain of any level because even a top-level domain is a "subdomain" of the "root" domain (a "zeroth-level" domain that is designated by a dot alone).

Traditionally, the second-level domain has been chosen based on the name of a company (e.g., centerzerocompany.com), product or service (e.g., centerzeromail.com). The third level was commonly used to designate a particular host server. Therefore, ftp.centerzero.org might be an FTP server, www.centerzero.org would be a World Wide Web server, and mail.centerzero.org could be an email server. Modern technology allows multiple servers to serve a single subdomain, or multiple protocols or domains to be served by a single computer. Therefore subdomains may or may not serve any real purpose. 

Official assignment

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 any.example.edu and any.other.example.edu to interested parties.