In computing, promiscuous mode or promisc mode is a 
configuration of a network card that makes the card pass 
all traffic it receives to the central processing unit 
rather than just packets addressed to it — a feature 
normally used for packet sniffing.

Each packet includes the hardware (Media Access Control) 
address. When a network card receives a packet, it checks 
if the address is its own. If not, the card normally drops 
the packet. But in promiscuous mode, the card doesn't drop 
the packet, thus allowing the computer to read all packets.