Subdomain
From Free net encyclopedia
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. A DNS hierarchy consists of the root-level domain at the top, underneath which are the top-level domains, followed by second-level domains and finally subdomains.
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.[1] 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"[2], 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."[3] 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, itself.).
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'.
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Length
The length of a particular subdomain label is restricted to 63 characters, the number of subdomain labels in a domain name is limited to 127, and the sum of the lengths of all labels must not exceed 255 characters.
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.
Real life 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.
Subdomain URL shortening services
There are many URL shortening services on the Internet. These services provide redirection from a long URL, such as from http://www.example.net/members/3525/main/page1.html to something more like 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.