I have a linux gateway for my internet connexion ppp0 that is connected to two computers through two network cards, eth0 and eth1. So i have two subnets.
I've activated the masquerading in iptables.

If one of the two computers run a soft like Yahoo Messenger, MSN, or ICQ, and one other from internet want to send him a message, the message will arrive on the ppp0 connexion of the gateway.
But what can the gateway do with this message ?
How the gateway can decide which computer this message is destinated ?

Is there an Iptables rule ?

Thank you for your help.

Exalik

I don't know that I understand your question, but...

The Iptables MASQing rule takes are of any connection to ppp0 that is actually destined for one of your messenging software clients on either eth0 or eth1. It does this by modifying the destination address of the packet as it is processed by the Linux machine.

Are you having problems with something specific that you can describe, or are you querying the inner workings of iptables?