[AccessD] External IP Address of Local Machine

I am using WDSI, WinInet and various API's to get 2 Access applications
to send data to each other over FTP.  It is working well, but I want the
server to be secure, that is to only allow logins/files to come from
specific IP's. This is ok when I am sending, because I know my IP, and I
tell the client to allow my IP.  When I (hopefully) sell a copy to the
client's client, how can they easily determine their IP?  
I have looked at the Winsock API and can get remote IP's via Host Name.
However, when I want to get the IP of the local PC, it returns the
192.168.x.x address; how can I get the external IP that the FTP server
needs to know through VB/API?

TIA,
Steve