Let's imagine that we have a company, called mycompany.com. At our head office, we are using the 192.168.0.0 reserved network, breaking the class B into 256 class C networks to allow routing. We have just set up two small remote offices, and want to add them to our network. We also want to allow employees who work from home to be able to use their DSL and cable modem connections instead of making them use dialup. To start, we need to plan things out a little.

I decide that I want to give each remote office a class C network range to allow them to expand as necessary. So, I reserve the 192.168.10.0 and 192.168.11.0 nets. I also decide that for home users, I've got enough numbers that I don't need to masquerade them on the VPN server side. Each client gets it's own internal IP. So, I need to reserve another class C for that, say 192.168.40.0. The only thing that I must now do is to add these ranges to my router. Let's imagine that our company owns a small Cisco (192.168.254.254) that handles all of the traffic through our OC1. Just set routes on the Cisco such that traffic headed to these reserved nets goes to our VPN server (192.168.40.254). I put the VPN server into the home user's net for reasons that should become clear later. We'll name the external interface of the server vpn.mycompany.com, and the internal vpn-internal.mycompany.com.

As for external numbers, we don't need to know them explicitly. You should have your own numbers, supplied by your ISP.