Forum Forum URL Problem

[محروم.كوم]

My forum is installed in the root directory. When setting up the forum, I had to use http://mysitename.com as the Forum URL. If I added "www." such as http://www.mysitename.com the forum did not work?

It works fine now, however if someone types in "www." and then my forum name, the browser generates an error that it could not find the site? How can this be fixed so the site will load either way as all normal web sites do? __DEFINE_LIKE_SHARE__