If you are using a router try the following:
Disable any firewall or security features in your router
-Set port forwarding on your router to the IP address of your Playstation 3. Call of Duty: World at War uses ports 3478, 3479, 3074, 3075 (UDP) and ports 80, 443, 3074, 5223 (TCP).
-Place your Playstation 3 into the DMZ of your router.
If you need help, go to PORTFORWARD.COM
this information was provided by JIM GILL. if it works please rep him for his work.
thanks