Once two NDI devices have discovered each other on the network, video can be passed from the sending device to the receiving device. After the compression of the video, the NDI sending device opens a session to the receiving NDI device. At this point, we have two endpoints that consists of an IP address and a port number.
The NDI network frame is a TCP packet that transmits in the Ephemeral port range. Ephemeral ports are temporary ports assigned by the NDI sending machine’s IP stack, and are assigned from the designated range of ports for this purpose. When the NDI sending device terminates the transmission session, the port then becomes available for reuse, although most IP stacks will not reuse a port number until the entire pool of ephemeral ports have been used. This is important to keep in mind in the event of disconnection due to shut down of the NDI endpoints. The new connection will almost always be assigned a different port number.
The port ranges used for NDI are 49152 to 65535. If NDI devices are used on networks with firewalls positioned on the internal network, configuration to allow for traffic between devices in this port range is required.
You can view the Ephemeral port range on a computer that is running Windows Vista or later by use of the netsh command. These commands also provide a way to limit the port range, however this will impact all dynamic port assignments to that particular machine.