RedHat Firewall Port forwarding and VPN

Hi

I am trying to setup windows 2000 vpn server behind my redhat linux firewall. I have a cable modem with static IP Address. I am alreay running Mail, web server behing this firewall. I open port for windows 2000 vpn server on my firewall(Port 1723) I had no luck. Can anyone help me i know I have to do soemthing with IP ID 47 but I dont know how to setup. I tried setting up 47 as a regular port on my firewall but no luck. Can anyone help me with this situation also please let me know if youguys need more information.

Thanks

Jay