What is a tunnel server

https://docs.arednmesh.org/en/latest/arednGettingStarted/node_admin.html...

Tunnels are typically used as a means of connecting mesh islands if RF links cannot be established.
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Any node hardware CAN be a tunnel client or tunnel server.  To use a tunnel, the node must have an Internet connect and the Tunnel Client will initiate a connection over the Internet to the Tunnel Server.

One note on my statement about needing an Internet connection.  It CAN be some other network beside the Internet.  But it does need to be some network that will route the connection for you.