Hi,
System Preferences > Security (and Privacy) > Firewall.
In Snow Leopard and Lion you need to unlock the padlock and then see if the Firewall is ON
(If Off you can stop looking here)
If On then go to the Advanced Button
In this pane the Allow Signed Software (Applications) needs to be Enabled and iChat needs to be in the list (It may already be there if the OS asked you to allow it)
That takes care of the Mac.
Your Routing devices may be another matter.
Depending on your device you may have several ways of Opening or allowing ports.
If your device has it use UPnP (Universal Plug And Play) as this will allow Screen Sharing in iChat and allow Multiple computers (or one computer to change IP now and then) to use the same ports.
If you have an Apple Base Station then enable Port Mapping Protocol in the NAT settings (Internet screen)
8:57 PM Tuesday; November 22, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously