The network interface is the network card which the probe is currently collecting all the network traffic from. To change the network interface, click the stop button, change to the card you want to be probed, then click the start button. Uncheck the "Promiscuous" box if you want to start the probe in non-promiscuous mode. In non-promiscuous mode only traffic going from/to the probing computer will be picked up, as well as broadcast/multicast traffic.