The only reason you can't route private addresses across the Internet is that all ISP routers drop packets sent to these address ranges.
Right.
You would set up your VPN system to forward these packets, just the same as you can set up your own firewalls and routers to forward them if you want to.
A VPN with two RFC1918 ranges at each end is a very common setup.
Yes, indeed.
My question, though, is how can a connection be established between two parties where one of them has a private address (A) and where you want to connect _to_ the server having the private address (A, see below).
The problem is, you can't establish a connection to the private address (A), so there has to be a means of 'hijacking' the established session (from X, see diagram below).
(Internet) (Internet) A <------------> X <------------> B
A: 192.168.X.X B: 192.168.X.X X: public IP address
The end result is to get from B to A, securely.
Cheers Sven
