Opening ports on Firewall for an specific IP

Hi,

First of all, I apologize if the answer for this is somewhere else. I have searched the forum and couldn’t find anything.

We have a small network that uses a Mac OS X Server 10.2.8 to serve DHCP, NAT and Apache. It also works as a firewall. Recently, I have been asked to block MSN Messenger access to all users on the network, which as pretty easy (I simply blocked all the ports on the Firewall for Messenger and it worked fine). Now they are asking me to allow some users/IPs do be able to connect to MSN Messenger. Is there a way of doing this? Can anyone point me to a tutorial or a book where I can learn how to do it?

Thanks a lot in advance,
Thiago