NAT support - how to obtain the public IP address?

Hi
I am developing an instant messenger in which users can start a conversation with another user through obtaining the IP address of the intended recipient from a mySQL database on the web server.
This works fine within a local network. However, the address in the database upon user login is the private IP address within the LAN and not the global address - consequently my software cannot be used outside the LAN currently.
Is it possible to use a method within the java.net library to send the public (global) address of the client to a server? And if so, how can I handle the ability to receive a reply from the server (which would arrive at the public IP the message was sent from, ie. the NAT firewall) so that it is delivered to the correct port and private IP on the client?
Joe Barber

If the server is on the public internet and the client is on a private network the server will see the public address and port, and can get it from a connected socket. Unfortunately if the nat box uses DHCP there is no guarantee that this will be the same next time, so persisting that value in a database is not a good idea. If both the client and server are behind nat gateways, they cannot directly establish calls. The better choice for these kind of systems is to run the server on the public internet on a well known address and have your clients connect to the server.
You will find lots of discussion on the difficulties of nat and p2p on this forum and by looking in google.

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