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A subdomain, occasionally referred to as a "canonical," and sometimes called a "third-level" domain, is another URL address that can be created instead of a Web site subdirectory. Whereas, a particular web page domain is a folder that contains the site files; a subdirectory is a folder contained within this main folder. A subdomain, on the other hand, is basically an alias, as any Internet user can enter the subdomain in a browser's address bar to view the subdirectory with which it's associated.

A subdomain combines a unique identifier with a domain name to become essentially a "domain within a domain." The unique identifier simply replaces the "www" in the web address. Yahoo!, for example, uses subdomains such as and to reference its mail and music services, under the umbrella of[1]

Unique identifier

This table shows the structure of a subdomain.

http:// subdomain . domain . com
3rd level 2nd level 1st level

The logic is that ".com" is the first level domain, "Conservapedia" (etc.) is the second level, and "www" or "http://" is therefore the third level. Thus, the domain name "" is therefore a "real" domain name; however, "" would be a subdomain.

Machine names

Subdomains are sometimes used to identify individual computers, each having an IP address of its own. In that case, the term "machine name" is sometimes used instead of "subdomain." For instance, a company could have two servers called "" and "" The two computers would serve different departments, and contain different information, but share the same domain name.[2]

Organizational tool

In most cases subdomains are used to organize information with a website. For example, an on-line shop could use the subdomains "," and "," etc., to store related information together. An excellent example of a site that uses subdomains for this purpose is

A Web site that uses subdomains could have used subdirectories, for example "" instead of "" But the use of subdomains allows for a company or the host of a domain to more easily move a topic to a dedicated computer if it turns out to be very popular. The larger a site is, the more likely this "scalability" could be an important benefit and factor for using subdomains over subdirectories. For smaller sites, however, subdirectories work fine; it may not need the two-hundred or more subdomains included in a hosting package. Subdomains are available only in Web Hosting and Merchant Solutions plans.


  1. What are subdomains?, Yahoo!
  2. Subdomains, Web Hosting