Subdomain

In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. For example, "example.com" is a subdomain of the "com" top-level domain (TLD) and "www.example.com" is a subdomain of "example.com". In fact, the "com" TLD is a subdomain of the root domain.

A subdomain is sometimes termed a "vanity domain", especially when it is a subdomain of an ISP's own domain aliased to an individual user account. However, the term "vanity domain" has other usages:

• the Free On-line Dictionary of Computing defines it as "A domain you register for the sole purpose of having your own domain so you can have an easily remembered URL and e-mail address" [1],
• while the Jargon File defines it as "An Internet domain, particularly in the .com or .org top-level domains, apparently created for no reason other than boosting the creator's ego." [2]
• Everything2 defines it as an "abuse of the DNS system to spell out sentences or messages" [3] also referred to as a domain hack.
• The Sun iPlanet Messaging Server uses msgVanityDomain to set up a domain name for e-mail use which is not fully hosted.

These uses apply primarily to regular second-level domain registrations rather than subdomains (although, technically speaking, a second-level domain is actually a subdomain of its top-level domain).

Valid subdomains as described by RFC 1034 may contain any letter or number as well as the '-' (hyphen), but must begin with an alphabet character. While both upper and lower case characters are valid, subdomains are case insensitive, so 'ABC.com' is identical to 'abc.COM'.

Structure

Subdomains are separated by dots "." and are read from left to right, in descending specificity. Each consecutive subdomain contains all previous subdomains, and is contained by any subsequent subdomains. All subdomains are contained by the second-level domain, which is in turn contained by its top level domain.

Example

subCBA.subBA.subA.domain.tld

In this example the top level domain is 'tld' and the second level domain is 'domain'. Within 'domain' the subdomains are contained in a tree structure.

Actual usage

Popular real-life examples of subdomains include news.google.com, mail.yahoo.com, and en.wikipedia.org, in which news, mail and en are subdomains, respectively.

However, many companies and organizations decline to use subdomains in a logical manner to address their departments, branches, chapters, and subsidiaries, choosing instead to use separate domains that they regard as better for marketing purposes, such as abcnews.com for the news division of the ABC television network instead of a more structured address such as news.abc.com. Some have pointed out that the use of such addresses encourages phishing schemes, which sometimes use plausible-looking domains that seem like they might be associated with a trusted company or organization; if official sites stuck strictly to subdomains of the main domain of the organization, then such schemes would stand out better to those who are attentive to the addresses they are accessing.

Some of the new TLDs have as part of their explicit purpose and business plan an intention to encourage organizations that already have a site in a different TLD to register in the new domain for specific sites with a particular aim -- .jobs for job postings and .mobi for mobile-device-accessible sites, for instance. Some critics believe that these aims would better be served by subdomains of the organization's original domain, such as jobs.company.com.

There is some speculation among search engine optimizers over what effect the use of subdomains may have, with some believing that they may be penalized in search results. There are also differing beliefs as to whether subdomains are considered to be part of the same site as their parent and sibling domains or different sites, for the purpose of calculating site rank.

Subdomain URL shortening services

There are many URL shortening services on the Internet. These services provide redirection to a long URL, such as http://www.example.net/members/3525/main/page1.html from a shorter one, for example http://name.ly2.com/. URL shortening services are usually free and come with ads. Most of the URL shortening services have URL cloaking, which creates an invisible frame to make the URL name in the browser to stay the same. URL shortening services may or may not have website tracking.

External links

• Domain Names - Concepts and Facilities
• Domain Names - Implementation and Specification
• DNS Mapper Tool - Wordlist based tool for searching for subdomains

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