I believe it's to confirm your IP address. The IRC server sends an ident message to your IP address, and the response it gets confirms the address. When the ident request comes in on a particular port, the proxy doesn't know which computer is listening on that port, doesn't know where to send it, so it dies. I recall that the NAT32 manual, when I last read it many months ago, had a discussion of the issue, and the NAT32 web site had a link to an identd program that could solve the problem. An alternative would be to set the IRC computer as the DMZ computer, which would allow it to get all unknown port packets.