This refers to the IP Address of the machine where TFTP Server is running.

    For downloading config files in the same LAN, leave this as it is.

    For downloading the config files of the devices in across WAN, specify the NAT IP here. 
    The TFTP Server will, by default, be bound to the local IP Address of the machine where
    OpUtils is installed. Since this IP cannot be reached directly, you need to specify a NAT IP
    that can be reached across WAN, which will be translated to the local IP by the router.