I had to do something similiar 20 years ago:
A large financial institute bought another.
Both institutions used - purely by chance - the exact same IP range, even this was an unusual range, so that they used the same network was like a lotto winning.
The network was something like 172.17.111.0/24 (on both sides).
Licensed software, also services like Bloomberg / Reuters prohibited changing IPs…
I had to connect both networks.
At the time, this was a BIG headache.
I used a somewhat tricky solution: A 2-way 1:1 NAT, essentially mapping every IP in 172.17.111.0/24 to a corresponding IP in the 172.17.112.0/24 subnet. From then on, an IPsec VPN from
172.17.111.0/24 <-> 172.17.112.0/24 did the trick.
IP routing made all hosts available to the other side.
From then on (working VPN between the 2 networks) we then secured both networks from each other…
As in some games tips: “There be dragons here”…
I’m glad it worked, and then long and stable enough until a real integration (New IP range) was possible.
My 2 cents
PS: A issue with “bridging” is often forgotten:
Quite often, IPs are used on both side, often enough exactly those can’t be changed. Watch out for IP conflicts! (Same IP used on both sides).
SoftEther sounds interesting, I’ve never heard about it. However, there are quite a few security “gotchas” here, besides language issues. I absolutly don’t need chinese or japanese for my clients. I’d need german or french. Chinese or Japanese characters would make my clients freak out…