Zerotier is a great open source project that enables a Global Layer 2 network.
ZeroTier, OPNSense and FRRouting
Well written Blog on this topic:
I am a huge fan of the work the guys over a ZeroTier have done in simplifying virtual networks on a Global scale with their Virtual Layer 1 and Virtual Layer 2 networks.
This has made some of my projects much more simple and makes the internet work the way it was supposed to when it was designed as a peer to peer network. It is a good bridge to enable some of the benefits of IPV6 all while still running an IPV4 private network. I will continue to follow this team as they grow this product's features. I just hope that they stay open source so that everyone can use this technology.