The code is all open source, and you can self-host the controller or use the ZeroTierOne service with either free or paid plans. Their free plan (cloud based) is robust, solid, and very consistent.
After you have two nodes connected, connecting to each other — no matter where you are or what side of any firewalls you may be on — is exactly what you would expect if you were in the same building on the same network. A primary use case is for remote access to a Home Assistant setup without needing to open up firewall ports or expose it to the internet.
The linked article explains how to get going with it.
See
How I manage my own virtual network with ZeroTier#
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networks 
Automation is a hot topic right now. In my day job as a site reliability engineer (SRE), part of my remit is to automate as many repeating tasks as possible.
