"Unidentified Network" (private Ethernet treated as "Public")
A large percent of modern computers end up with more than one wired network adapter port. One port will connect to the general network that connects to your other machines and to the internet. Rather than letting the extra ports go to waste, you can connect two machines directly to each other with just a cable, or three or more machines can have their own private network using a dedicated switch (or VLAN if you get fancy). The problem is that Windows by default classifies this network as “Public” and assigns restrictive Firewall rules that you might intend to use on an unprotected network. If you manually change this network to “Private”, it reverts back to “Public” every time you reboot a computer.
In addition to networks with no Gateway, this also affects a Windows computer that is itself acting as a Gateway, sharing its own network connection with one or more devices connected to it on a secondary wired port.
Windows assigns the classification of “Unidentified Network” to any network adapter connected to a network with no router that provides automatic configuration and gives access to outside networks. In technical terms, a network adapter with a static IP address but no gateway address is on an “unidentified network”.
It is difficult to image a case where any such network is public and has uncontrolled access from potentially dangerous machines, but the Windows default is to reset the classification of such a network to “Public” every time the system restarts.
There is a Group Policy that can override this default behavior. This is one case where the default behavior is so bad that you might consider routinely changing it on all your computers, so you do not have to diagnose connection problems after you make a connection between machines with unused network ports. Run the Group Policy Editor utility (gpedit.msc). Navigate to the policy shown in the diagram below, and switch the Radio Button from “Not configured” to “Private”:
