How to block yahoo messenger chat?

[sim]

I have a users inside my nework who used yahoo messenger
and msn to chat in the internet. I have a firewall inside
my network but my problem is I can't find the IP address
of the server that yahoo used for thier chat software. Can
anybody tell me on how to block these users from chatting
over the internet within office hours? Thanks a lot!

 

[Jayson Ferron]

you can do a netstat -a -n on a work station or block the url on outgoing
connection on your fire wall