Poorly worded requirement which supposed a routing solution vice explaining the functionality desired. Why do you need to frig with ports and why.
I wish to provide my server for friends which hosts on port 666 but my ISP blocks 666.
I cannot change the host port (fixed port) of my satan server.
I have a limited number of users and I have their public WANIPs if required.
They will use dyndns to access my server dydnsname is "lucifer"
However, without the context of a good requirement, (what we wish to accomplish in plain words and not in not router programming terms, and why) trying to understand or answer contortions on ports and subnets is not fruitful.