|A packet sniffer is a wire-tapping devices that plugs into computer
networks and eavesdrops on the network traffic. Like a telephone
wiretap allows the FBI to listen in on other people's conversations,
a "sniffing" program lets someone listen in on computer conversations.
The ethernet was built around a "shared" principle: all machines on a local network share the same wire. Thus all machines are able to see each other.
Ethernet hardware is built with a "filter" that ignores all traffic that doesn't belong to it. It does this by ignoring all frames whose MAC address doesn't match.