When ZoneMinder communicates with MPEG4 capable cameras using RTP with the unicast method it must open ports for the camera to connect back to for control and streaming purposes. This setting specifies the minimum port number that ZoneMinder will use. Ordinarily two adjacent ports are used for each camera, one for control packets and one for data packets. This port should be set to an even number, you may also need to open up a hole in your firewall to allow cameras to connect back if you wish to use unicasting.