A common question we get from customers that use Community Server is,

"When I make a request to my Community Server site, the "www" is stripped off the domain..."

While it's easy to change this behavior in Community Server, it's actually done on purpose to help ensure the site is optimized for search engines. A search engine actually sees www.example.com and example.com as 2 separate sites. Content on these 2 sites is considered duplicate, and the site will actually get a penalty in search rank (in some cases).

There are some more interesting tips--similar to this one--in this article.