server only listen localhsot. WHY???

Good day,
I got some problem for setup a ftp server.

I use 'serv-U FTP server'. The problem is my ftp server not listen to public IP( only listen to local lan IP and loopback IP).

I using a alcatel speedtouch port router to connect to my local ISP. My ISP provide me dynamic public IP .My configuration is something like this
My Pc lan card(192.168.0.3) coneect to that alcatel router(192.168.0.1).But my ftp server only listen to 192.168.0.3 and 127.0.0.1....NOT listening to public IP address.


Please let me know what I need to configure so that my "serv-U ftp server" can listen to public IP address.The ftp server is behind a router. What I need to do so I able to setup my ftp server with my Alcatel speedtouch st-510 router???


Detail:
OS window Xp Home
FTP server software serv-U FTP server

Remark: The serv-u ftp server can work fine when I use a 56k modem to dial to internet.Is no point I setup a ftp server with 56k modem. I want to setup my ftp server with ADSL line and with that alcatel.

Thanks so much in advance....your feedback will be so appreciated.

you need to redirect port 20 and 21 (default ftp port) at the router to you servers ip (192.168.0.3)