Mixed OSs on a LAN

[Zelmo]

I have a LAN that my friends and I use to play Quake III. Until now, everyone has been running Windows 9x and happily fragging. The other day, though, a new guy came along with a Mac and we want to get him on the network when we play. He has the TCP/IP protocol, his network switch light comes on when we plug him in, I can ping him successfully from my Windows 98 box, but when he starts up Quake, he can't see the server. I know file sharing isn't an option with our equipment, but don't people with all different platforms connect to internet Quake servers on all different platforms every day? How can I bring that to my LAN and make the Mac frag-happy with the PCs?