This is what the NETMAP target in iptables is for - map an entire subnet to another subnet, including the reverse. We were doing this 20 years ago for clients t

This is what the NETMAP target in iptables is for - map an entire subnet to another subnet, including the reverse. We were doing this 20 years ago for clients trying to on-board other companies that they'd bought. It's horrible, but it does solve the problem in a pinch.