2ner 0 Newbie Poster

Setting up my comp at work to host Remote Desktop Connection, on the SpeedStream 6300 modem/router I try to Port Mapping Setup > Select Protocol: > TCP > and TCP/UDP port(s): >3389 - 3389
It already has my comp's correct static IP in the field below.
I get error message:
NAPT server IP address is not a valid host LAN address.
I have windows firewall off, zone alarm allowing in and out TCP/UDP 3389, remote desktop enabled in system > remote. What am I doing wrong?
Any help appreciated.