All a sub domain is, is a folder that the server sets an active link to and makes available at blan . yoursitename.com
heres some useless knowledge, www is a subdomain, give it the most common subdomain in history, but thats all that the www before a site name is, it was created to be a commonly used subdomain early on and now is almost the default, however most sites can be accessed by going to the site name without, but it is possible to have 2 totally different sites at
www.mysite.com and
http://mysite.com ...like I said useless knowledge, oh well.