A subdomain is any domain which appears to the left of a second-level domain in a URL. For example, the "things" in "http://things.mystuff.com" is a subdomain, as is the "www". Once you have a domain name (e.g. "mystuff.com"), you can ask your host to add as many subdomains as you like.

Subdomains can be used for all sorts of purposes. The most common use of subdomains is to organise a website into sections. If a site is of a reasonable size, it can be useful to divide it up into separate subdomains containing different sections or departments of the website. For example, one might have "things.mystuff.com" and "widgets.mystuff.com". This use is as an alternative to having directories such as "mystuff.com/things/" and "mystuff.com/widgets/", and in cases like this it is mostly cosmetic.

Subdomains can have more practical uses as well. For example, if a site is very large, like About.com, different sections of it may be stored on different servers, and using separate subdomains for these sections can facilitate this. Organising a site using subdomains even for a smaller site is a good idea, because it allows for expansion: if a section gets more popular it can easily be moved to a dedicated server, whereas if a directory is used, this change will be much more difficult to implement.

Also, a site which receives a large amount of traffic might use "www", "www2", "www3" and so on, each subdomain containing the same information. A visitor who typed in "www.mystuff.com" might be redirected to "www5.mystuff.com", which is at the time handling less traffic than "www.mystuff.com".

Another use of subdomains is to separate different language versions of the same website. For example, Wikipedia uses "en.wikipedia.org" for its English language site, "de.wikipedia.org" for its German site, and so on.

You should note that the "www" sublevel domain is popular, but not actually necessary! Many websites are nowadays skipping its use altogether (such as del.icio.us and ma.gnolia.com), just having http:// and then the second-level domain. It's a good idea to have the "www" subdomain redirect to your website if you choose not to use it for your main site, as many people will type it in as a matter of course, but not using it can make promoting your website easier – there is less to put on business cards, posters and other marketing materials. Indeed, there are a few groups on the internet campaigning against the use of the "www" subdomain.

By Natalie Catchpole