Thank you for your message.
The Internet ID is generated by the Host itself, not the GMS. The GMS is only an intermediary that uses the IDs supplied by Hosts. The IDs are unique and there is very little chance that on the same ID server, especially if it's a local GMS server, there will be two identical IDs.
Strictly speaking, when you use the Internet ID connection and both computers - Viewer and Host - are on the same network, the program automatically detects "direct availability" of the Host and tries direct connection instead of the Internet ID connection.
However, the pre-requisite for direct connection is that incoming port 5650 on the Host should be open, i.e. allowed in whatever firewall software is installed on the Test PC. It's the default value, you can change the port in the Host settings (Settings for Host -> Options -> Network).
Hope that helps.