Monitor mode, or RFMON (Radio Frequency MONitor) mode, allows a computer with a wireless network interface controller (NIC) to monitor all traffic received from the wireless network. Unlike promiscuous mode, which is also used for packet sniffing, monitor mode allows packets to be captured without having to associate with an access point or ad-hoc network first. Monitor mode only applies to wireless networks, while promiscuous mode can be used on both wired and wireless networks. Monitor mode is one of the six modes that 802.11 wireless cards can operate in: Master (acting as an access point), Managed (client, also known as station), Ad-hoc, Mesh, Repeater, and Monitor mode.Some uses for monitor mode include: geographical packet analysis, observing of widespread traffic; esp. for insecure channels (such as through WEP), and acquiring knowledge of Wi-Fi technology through hands-on experience. This mode is also somewhat useful during the design phase of Wi-Fi network construction to discover how many Wi-Fi devices are already using spectrum in a given area and how busy various Wi-Fi channels are in that area. This helps to plan the Wi-Fi network better and reduce interference with other Wi-Fi devices by choosing the least used channels for a new Wi-Fi network.
Software such as KisMAC or Kismet, in combination with packet analyzers that can read pcap files, provide a user interface for passive wireless network monitoring.