IChat AV with NAT and more than one Mac

Question

I am a bit confused about firewall settings for iChat AV. I have a netgear FVS328 router. Obviously to have iChat working, I first open the firewall on my mac (Tiger) for the appropriate ports. Then I defined a service on the router and a rule to open those ports. It works... however, I have an issue.
In the router firewall setup I had to give an LAN IP Address to which packets for those specific iChat ports would be forwarded. Now, I understand this when a connexion is ignited from the Internet and (for example) you would want all HTTP trafic on port 80 to go to your web server. But here I have more than one mac behind my firewall. Moreover, they get there address by DHCP. I can't change the firewall setting each time I want to chat from a different computer!
I tried to have a look at port triggering, but it doens't seem to exist on the netgear. But even though, I would have to specify an IP address on my LAN.
Why is this needed since basically both iChat parties are already connected to .Mac so the connexion is made!
I am also wondering how they manage that kind of problem in Internet cafes where they certainly do not have a fixed Internet IP address for each PC or Mac available!

Answer 1

It is not possible to port forward the same port number to more than one computer on any router that does NAT. It is impossible for the router to know which of the computers to send it to and sending it to both would confuse the software.
Think of it another way all post for a particular house is delivered by a postman. Once posted in the mailbox someone picks it up and ensures it reaches the correct person by the name written on the post. Now if you have 2 John Smiths in the same house nobody will know which one it is for. The same applies to the port numbers on the router. Some applications allow you to select a unique range of ports for each computer so you can organize a unique port for each computer but iChat doesn't.