Yesterday it worked.
Today it is not working.
Windows is like that.
So are networks, oddly enough.
There are many many ways you can set up a tunnel between two networked hosts. The MetaNet is one such example of a tunnel - it is a multi-point tunnel.
Most tunnels are point-to-point. Some examples are:
You may want to authenticate your users against another machine; look at RADIUS and TACACS.
One page links to TunnelNotes: