Easiest way is to probably call your ISP. You could try the port forwarding link from a couple posts up if you don't want to call them and see if that fixes your issue, but if your ISP is restricting it, port forwarding won't do a whole lot.
I did port forwarding with the help of Xbox Live Tech Support (via Twitter) a few months ago. It stuck for a week, and that was the last solution in a long line of them. I'll call Comcast (barf) when I get back from camping.