There are two distinct meanings of the word subdomain when discussing domain names: one technical meaning and one popular meaning. It’s important to understand both so that when others speak with either one in mind, you’ll know what they mean.
Technically, in the Domain Name System (DNS) a subdomain is any subdivision of a larger domain. In this understanding, in this understanding, because the top-level domains (TLDs) like .com and .net divide up the Internet, they are the highest level subdomain. Domains like amazon.com or us.gov and the next level of subdomain, and so on and so on. It is conceived of as a hierarchy, like the hierarchy of divisions on your computer hard drive.
In popular parlance, subdomain refers specifically to the level of division after domains. Coming after TLDs and domains, subdomains forms the third level of the hierarchy. Notice that in Uniform Resource Locators (URLs), the TLDs are on the far right, and each subsequent level down is one step to the left. So you would expect to find subdomains to the left of the domain name. Some people use the term vanity domain to refer to a subdomain. Especially because the term vanity domain has a particular meaning with regard to unintended use of Country code TLDs (cTLDs), it is suggested that the names be kept distinct.
What’s a Subdomain Good For?
With a subdomain, you can clearly indicate important subdivisions of your site, like departments, functions, or services. For example, universities might use subdomains for their major divisions, like humanities, social sciences, and physical sciences, like this:
An online seller might distinguish their company information from their forum from their webstore, from their affiliates program, like this:
Subdomains can also designate individual host servers, and subdomains such as ftp and mail do this, in this way:
Different server clusters can be specified as well, using discriminators such as www2, www3, etc
Note that it is possible to construct website divisions with paths that follow the domain name and which are divided by slashes, like this:
Subdomains can also be used to allow separate site sections for different members of an organization or family, or even to create separate sub-sites.
Where to Find a Subdomain
In some cases, when you arrange with a host company to set up a website, you are given space to create your site on a subdomain of that company. This is true, for example, with Apple’s .mac websites. If you wish to have your own site name, then this choice may not serve you well. In addition, although subdomain hosting may be offered for a low price or even be free, there may be restrictions on bandwidth, site space, or even the types of use to which you may put your site that may reduce the initial attraction. Watch particularly for subdomains that may not have commercial content.
If you want to have your own domain as well as subdomain(s), you will need to register a domain name. If this is one of your goals, be sure that the web host that you contract with offers and supports subdomains.