Increasing security for my Home server

[wenzelja]

On most routers it allows you to provide an external and internal port.  You'd change the external port to something else, but leave the internal port on 3389.

So I looked at my router under port forwarding and there is an option checked to make external and internal ports the same. So, do I just assign random port numbers to correspond with the internal ports, in order of the string of internal ports, I suppose?