We have the situation where we need to network an isolated room in a building to a network on the other side of the building but with an 3rd party company owning all the rooms between them (they lease us our rooms). The building is 300 years old and has thick stone walls not conducive to new cabling or Wi-Fi.
At some point, a CAT5 network was installed in the part of the building we need to span, and this network happens to reach nearly all the way across the building. We want to join our "room A" and "room E" networks together, without giving the company that owns the rooms the middle (B,C & D) any visibility of our network (or vice-versa). Clearly the best solution would be to run a new cable through the other company's office, but in practice this will be very disruptive in our 300 year old building, so I'm wondering if there's a way to utilize their cabling in a secure way (for both parties).
Are there any devices which would allow us to create a private VPN which runs over the other company's network, without ever exposing either network to the other? Essentially what we're trying to do is a bit like the idea of using HomePlug adapters, but using someone else's CAT 5 network instead of mains cable.
One thing which help: We have a Fortigate firewall which could potentially act as a VPN host is in a room which adjoins their network.