Obtaining client IP address

Hi,

I need to have a db table containing all connected clients (of a
distributed application) and among other info, their IP.

If both the client and the server are on the same LAN, no problem, the
client can supply it's own info. But if they are connected via some sort
of bridge/router/NAT? For instance, a client on a LAN connects to a
server over the Internet?

The client has no idea what public IP it has. The server ought to
know... however I haven't found how I could get this info into my query.

What I need is some sort of procedure whereby the client enrolls onto
the system and supplies the info...

I hope I made my self clear. If not I can try and be more specific!

Alex.