Huh? Be more specific here... you talking a local "NAT" IP address or what? You need to port forward, that's the only way. How you sort out how to port forward and arrange your internal network is up to you.
DHCP reservation is the best, but even if you can't do that you can just set the given machine to a static IP (outside of the DHCP reservation range). Kind of a pain, but there's no getting around it here.