yes, I think you have it...since it's not a DNS/DHCP issue, it's most likely going to be ICS's problem.
http://www.practicallynetworked.com/support/icq_nat.htm
http://windows.microsoft.com/windows...onn_config.htm
http://www.duxcw.com/faq/ics/ics_icq.htm
seems like ICS isn't that good of a NAT implementation, a little too harsh...sorry, I've not played with it much, since it doesn't have the configuration or the stability I need, and I would really rather not place my network at the whim of a Windows box. it does seem that you're going to need to map ports, or reconfigure your client to use a firewall. hopefully, the client behaves exactly as a "real" ICQ client does, there seems to be a lot of information on the problem around. if you need to see what port it's trying to connect on, try looking at what ports are open with "netstat", or view the every packet that's being sent and recieved on your network in realtime with "ethereal", if you have it installed(if you don't, install it...).