The probe response received normally contains BSSID and WLAN SSID.
Passive scanning is not done by active probing, but by mere listening to any data sent out by the AP.
By impersonating this AP by automatic altering of the MAC address, the computer running the network discovery scanner will be given this SSID by legitimate users.
Passive scanners include Kismet and essid jack (a program under AirJack).
Notable programs include Network Stumbler, Kismet, Lumeta Corporation, Aerosol, AirMagnet, MacStumbler, Ministumbler, Mognet, NetChaser, perlskan, Wireless Security Auditor, Wlandump, PocketWarrior, pocketWinc, Prismstumbler, Sniff-em, AiroPeek, Airscanner, AP Scanner, AP Radar, Apsniff, BSD-Airtools, dstumbler, gtk-scanner, gWireless, iStumbler, KisMAC, Sniffer Wireless, THC-Scan, THC-Wardrive, WarGlue, WarKizniz, Wellenreiter, Wi-Scan and WiStumbler.