Zerotier

Zerotier is an open-source1 network virtualization platform.

How they describe themself:

ZeroTier is a smart Ethernet switch for planet Earth.

It’s a distributed network hypervisor built atop a cryptographically secure global peer to peer network. It provides advanced network virtualization and management capabilities on par with an enterprise SDN switch, but across both local and wide area networks and connecting almost any kind of app or device.