I dont think anyone mentioned the "behind the router" issue.
Depending on how your router works, you will want to "port foward" the FTP port on the router to the FTP port on your linux box. This port is USUALLY port 23. look at the documentation for you router on details on how to actually do the port forwarding.
Doing this will actually allow people from the outside to access the ftp server as if it were infront of the router, but will keep the server safely behind the firewall features of the router.
Make sense?