Hello,

Thank you for contacting Activision Technical Support. I am sorry to hear about the problem you are having. This issue is most commonly caused by one or more of the ports used by this game being closed on your router.  You need to forward the following ports through your router to your PC in order to resolve this issue:

Call of Duty: Black Ops transmits and receives network traffic on port 3074 (UDP and TCP). Additionally the Steam client transmits and receives network traffic on ports UDP: 27000-27015, 27015-27030 and TCP: 27014-27050

To check and see if these ports are in fact closed, and causing this issue go to the following link and put the above port numbers and click "Check" and this utility will notify you if the ports are open or closed.:
http://www.yougetsignal.com/tools/open-ports/

If you need any assistance in forwarding these ports you would need to consult your router manual or manufacturer and they would be capable of walking you through how to forward ports.

If forwarding the ports does not resolve the issue then set your PC as the DMZ host on your router and this could resolve the issue.

Please note Activision can not provide assistance in configuring your network. If you are unable to contact your Router's Manufacturer you can try visiting the following website:

http://www.practicallynetworked.com/