Change IP range, provide NAT for some old addresses. How?
I currently have a small IP range (192.168.100.0/24) that I need to expand. I was planning to use 172.16.0.0/23, doubling my address space (which should be adequate for my network).
We have a VPN connection to our local council, and they have firewall rules for certain IPs in our existing (192) range. Upon approaching them regarding changing the configuration in favour of the new range I'd like to use, I've been told the 172 addresses I'm planning to use are already in use on their internal system, and I should NAT the required addresses back to their original 192 flavour, to avoid requiring any system changes to the VPN setup.
Given that my experience of NAT goes no further than my router at home (I do know what it is and what it is for), how might I go about achieving this?
(The router than maintains the VPN connection is a Draytek Vigor 2820 and a Windows server runs my DHCP.)
Please, no suggestions of "just change your new IP range to something the council don't use". I get the distinct impression they're in no rush to change the configuration of the VPN and thoroughly intend this is all handled at my end of the tunnel. It is my intention to be prepared if this is the case.