Windows 2003 + RRAS

I am running a small (tiny) network, with one external IP address connected to a 2003 server running RRAS for NAT, and a few clients. I want to be able to point my external address to different clients by the port number.

ie w.x.y.z:80 -> Client 1 w.x.y.z:443 -> Client 2 (note this is just an example I know you can have a secure website running on the same box/service as a non-secure)

I know you can do this (quite easily) for 2 IP Addresses, but alas I only have one as stated earlier. Can anyone help?