There's a lot of ways to block....but first open your router using Http://192.168.1.1 and go to the Administration tab, activate the logs.
This will show you actual IP website addresses accessed, the newest ones are at the bottom. You can add these addresses to the Access restrictions Tab....if you need to know what they are, USE GOOGLE to get the "WhoIS" website and type in the IP addresses for more concise addresses.....
Now, if you're looking to block specific functions within a certain website like Yahoo, use the Port Blocking in the Access restrictions Tab.....IE ports 5100 to 5105 will block only the webcam, Port 5000 to 5010 will block Voice chat in Yahoo. If they have dynamic port switching like MSN messenger go to the Port Forwarding tab and Port Forward the range of ports to an unused switch like
192.168.1.[3]
[3] is not used on my network.....
If you want to find the servers, another (but outdated) way is to go to the Support portion of any messenger, ask for proxy/firewall settings, and those are the ones to use, they should include the http:// servername and also the port numbers.
I personally use the ports, allows access but stops webcam voyeurs.....