KISMET

Kismet is a console based 802.11 layer2 wireless network detector, sniffer, and intrusion detection system. It identifies networks by passively sniffing and can even decloak hidden networks if they are in use. It can automatically detect network IP blocks by sniffing TCP, UDP, ARP, and DHCP packets, log traffic in Wireshark/TCPDump compatible format, and even plot detected networks and estimated ranges on downloaded maps. 

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