What type of sites are these? I don't mean porno or stuff like that. Are they HTTP or HTTPS? HTTP comes in on port 80 while the HTTPS comes in on port 443. You may have something configured in your router.
Here is a link on how to set up routers. They have instructions for ALL brands.http://www.portforward.com
Go to the Routers section and find your router. Then continue. It will walk you through the process, step by step.