My ‘www’ urls all re-direct to homepage – How do I fix?

I imagine this is a simple fix, but I can’t quite figure out how.

The short of it is this:
If I write out a url like http://www.vivbizclub.com/products then the user is re-directed to the homepage as opposed to the products page.
If however I write the url like http://vivbizclub.com/products (w/o the ‘www’) then the user lands on the products page.

Is there a simple change I can make within the wp-admin or perhaps within the ht-access file so that I can write urls with or without the ‘www’ and when they’re clicked they’ll go to the appropriate page?