How to block yahoo mesenger/chat

Looks like Yahoo Messenger uses port 5050 as its default outbound connection port.
Block that port with whatever facilities you have (proxy server, firewall).

hope this helps

Yahoo messenger can also go through you normal HTTP proxy. So if you deny access out on the normal port your users can just change the yahoo messenger to use either your proxy server or just go out on port 80. You can try denying outbound connections to the yahoo messenger servers on any port :-)

Yahoo Ports:
If you are using Cisco you can block these ports for Yahoo. It should do the trick.
Access-list 101 deny any tcp any any eq 5000
Access-list 101 deny any tcp any any eq 5001
Access-list 101 deny any tcp any any eq 5050
Access-list 101 deny any tcp any any eq 5100
Access-list 101 deny any udp any any eq 5050

It looks like you could block Yahoo servers but this will also deny Yahoo web access.

Access-list 101 deny any tcp any host 216.136.225.12 eq www
Access-list 101 deny any tcp any host 216.136.175.143 eq www
Access-list 101 deny any tcp any host 216.136.175.144 eq www
Access-list 101 deny any tcp any host 216.136.175.145 eq www
Access-list 101 deny any tcp any host 216.136.226.209 eq www
Access-list 101 deny any tcp any host 216.136.226.210 eq www
Access-list 101 deny any tcp any host 216.136.227.166 eq www
Access-list 101 deny any tcp any host 216.136.227.167 eq www
Access-list 101 deny any tcp any host 216.136.225.83
Access-list 101 deny any tcpany host 216.136.225.48
Access-list 101 deny any udp any any eq 5100

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