A Computer Network is collection of computing nodes connected through a switch or hub. Data is sent from a system (sender) to another system (receiver). At the layer 2 of the OSI layer, packet is converted into a frame which has the Source Hardware Address and Destination Hardware Address. In Ethernet based LAN, MAC address is the hardware address. All nodes have NIC (Network Interface Card) connected to the port of a switch or hub and each node keeps listening to data on the network. If there are any frames which has destination MAC address matching to it’s NIC’s MAC address, the node picks the frame and sends the frame up into the OSI layer for further processing.
In promiscuous mode, a node puts it’s NIC in a special mode that listens and captures all traffic coming on the port that it is connected even, if the destination MAC address in the frame doesn’t match to it’s NIC MAC address.