A diagnostic tool for identifying a configuration of a private network that may disrupt operations involving communication between two devices on the network. The tool, when run on a device in the network, can identify a “double NAT” configuration in which the device may be separated from other devices on the private network by a NAT device. The tool, when run on a computing device, identifies a NAT device, such as a router, to which the device is connected. The tool then identifies a list containing one or more other devices that may be connected to the NAT device. The tool tests whether these other devices also perform NAT functions and are on the private network. Both the NAT device and the devices that may be connected to the NAT device are identified and a determination is made of whether those devices are on the private network by sending requests using one or more protocols that devices on a private network conventionally use but are not conventionally used by devices on other networks.