I have an IPSec gateway server (FreeSwan) running on a public IP address, and I want to make tunnels from machines that sit behind ADSLModems (in this case, a Nokia M1122.) Thankfully, it was a bit easier to acheive than I thought it might be, and you don't have to worry about NatTraversal at all.
This works because FreeSwan can identify its ends with the leftid= and rightid= parameters, without needing the IP address blocks to match.
If you have multiple networks that have the same numbering, you can't tunnel them all together to the same server - how would the server know which was which? Along with that, if you're using a network between a firewall and a DSL modem (the 192.x examples below), it will also have to have a unique IP address.
This setup is designed to allow traffic between an internal mailserver and an external web site, for the purposes of IMAP mail. You can, however, modify it to suit. The entire internal network can still access the machine the tunnel ends on, and you also have to configure a tunnel between the external IP of the firewall at the local site, or else the firewall can't access that machine (which makes using webmail internally a bit of a problem.)
Imagine a (reasonably standard) layout: (External IP M1122 192.168.1.254) - (192.168.1.250 Firewall 10.7.1.254). The hosting server is 22.214.171.124.
This end has to have some different IP addresses from the other end...
I came to this page for confirmation of following assumption:
leftsubnet might contain the leftnexthop and left.
e.g leftsubnet is 10.110.30.0/16 , leftnexthop is 10.110.30.1, left is 10.110.30.42
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