I want to create a virtualised WAN. As in, I want to have a couple of VMs (VBox) on one physical host machine, that exist on separate LANs, but that can talk to each other. Do I make the VMs, set them up with different IP addresses (e.g. 1.2.3.4 and 5.6.7.8) and then configure port forwarding between them somehow???

I've seen articles that set up port forwarding on port 2222, but I don't really understand why this works. How is setting up the VM to listen to port 2222 and then port forward from there to, say, port 80, any different from just telling the VM to listen on port 80 in the first place?

FYI, the VMs run Ubuntu Desktop 14.x.