Can't use www, I think it has to do with multi-site?

Hello,

I have a problem that I imagine is pretty simple to solve, but I’ve looked all over this form and Googled the question every way I can think of, and I’m not finding the answer.

If I type in the URL of a page on my site without putting www in it (e.g. mysite.com/foo), everything works fine, but if I type http://www.mysite.com/foo it redirects to mysite.com. In other words, I can’t access any page on myste.com other than the homepage if I enter www. This is a problem because I think I’m going to get a decent amount of type-in traffic and links that have “www” in it.

So how do I fix this? Is it a phpmyadmin db thing? An htaccess thing? A wp-config thing? I can’t find an answer anywhere. I strongly suspect that multisite is at least partly to blame.