By default when a network card receives a packet, it checks whether the packet belongs to itself. If not, the interface card normally drops the packet. But in promiscuous mode, the card doesn’t drop the packet. Instead, it will accept all the packets which flows through the network card.
Superuser privilege is required to set an interface in promiscuous mode. Most network monitor tools use the promiscuous mode to capture the packets and to analyze the network traffic.
Following will put the interface in promiscuous mode.
# ifconfig eth0 promisc
Following will put the interface in normal mode.
# ifconfig eth0 -promisc