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Date: Fri, 16 Aug 2013 18:17:52 +0100
From: Frank Leonhardt <freebsd-doc@fjl.co.uk>
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Subject: VPN where local private address collide
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Let's say we're using MPD on FreeBSD at both ends of a link here, using 
a VPN to connect two LANs. (The use of MPD is negotiable).

One LAN uses the address range 192.168.1.0/24 and the other uses the 
address range, er,  192.168.1.0/24. However hard you try to avoid this, 
it's going to happen. Let's also assume there is no way either end can 
change its range. NO WAY. Just don't go there.

I've found I can still get away with this if the actual used IP 
addresses don't conflict, but it's not ideal. I'm not even 100% on why 
it works at all.

I've heard of a mythical solution called "VPN NAT". It makes sense; just 
use NAT to map one range on to something completely different and away 
you go. Hosts at either end would be none the wiser.

Has anyone actually done this, and if so, how?

Thanks, Frank.