Yes, default port for RDP is 3389
So, when you go into Remote Desktop Connection Client, when you type in an IP address it automatically uses the default port 3389.
Luckily, you can change the port on the machines so you can have multiple machines using RDP on the same IP address, what you need to do is
a) Change the port number for RDP on each machine (google will tell you how, change to the registry). I would start with 3389 and go upwards from that amount.
b) set up the port forwarding on the router
c) when making the connection, append the correct port number that corresponds to the correct machine on the connection box
I assume you are OK with these instructions but if not then ask and I can explain in further detail.
Zen 8000 Pro