D-Link DFL-600 - Manual (Page 50)

Private IP This is the IP address of the server on your LAN
that will provide the service to remote users.
Transport Type You can select the transport protocol (TCP or
UDP) that the application on the virtual server
will use for its connections. The choice of this
protocol is dependent on the application that is
providing the service. If you do not know which
protocol to choose, check your application’s
documentation.