Entering now the firewall configuration problem... To use iChat AV, one of the two ends must accept incoming connections on an number of ports.
The Apple website gives some information about it but this isn't so clear about what ports are used for LAN chat or WAN chat and ends up with " If that does not work, try opening all ports in this range: 1024-65535". Yeah, sure...
So for your convenience, here are the required incoming ports and what they're used for (thanks to this page):
- 5060 - UDP: SIP (Session Initiation Protocol)
- 5190 - TCP & UDP: AIM/iChat file transfert
- 5678 - UDP: SNATMAP server (what's that?)
- 16384-16403 - UDP: RTP/RTCP (Real-Time Transport Protocol)