A virtual server presents a single IP address that represents two or more real servers. The virtual IP address is published to the user community, but the real Trailhead server IP addresses can remain unpublished, allowing you to hide actual site implementation details and provide a single point of contact for users. The virtual server address can be accessed only from the client side of LocalDirector. Also clients and the real servers bound to the LocalDirector virtual server cannot be located on the same side of LocalDirector.
In the Trailhead.server.properties file on the Trailhead server, set the value for the FormURL property to the virtual IP address for the callback form (callform.html).
Using the LocalDirector User Interface application, you must first create a virtual server and then create the real servers. Basically, you add the Name, IP address, Port, and bind ID using the Create Virtual Server window and the Create Real Server window.
See the Adding Virtual Servers and Adding Real Servers to Virtual Servers sections of the LocalDirector User Interface Install and User Guide.