P2P apps & filewalls

From: PJS (pj2k@hotmail.com)
Date: 12/14/01 

From: "PJS" <pj2k@hotmail.com>
Date: Fri, 14 Dec 2001 16:26:20 +1100

Hi all,

I'm just trying to figure out how these P2P apps work behind firewalls? My
PC is on a LAN, I have server connected to the Net which serves the LAN
Internet access using NAT software (also acts as a firewall). Kazaa seems to
work fine on the PCs in the LAN, I don't have to worry about port mapping or
anything. Instant messenger programs such as ICQ also work.

It is my understanding that when actual communication (file transfers etc)
take place, the two PCs talk directly to each other? If both PCs (peers) are
behind firewalls (running NAT), how do they initiate contact with each
other? All they would have is the IP address of each other? How does the NAT
software at each end know what to do with the initial connection requests?

Thanks.

PJS.

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