What is monitor mode?

I've seen monitor mode mentioned numerouse times but I'm not postive as to what it is.

Here's my interpretation:

If a card is not in monitor mode it will only see the traffic that is directly addressed to it (similar to passive mode with Ethernet).

If a card is in monitor mode it will see all the traffic on the network unless it is a switched environment (similar to promiscuous mode with Ethernet).

Is this correct or am I up in the night???

Also, are there wireless NICs that are automatically put in monitor mode or do commands have to be issued first?

Thank you.

I would add one point to your interpretation: In monitor mode, the card is only listening. It will not associate with a network.

You can assign monitor mode on boot, or allow an application such as Kismet to place it into that mode. Default mode is typically managed (infrastructure).

Thank you for the reply.

I'm guessing my interpretation is fairly accurate then along with your addition?

Yes. Here's a better wording of it than I can offer:

"Having a wireless card that operates in monitor mode allows the card to capture packets without associating with an access point or ad-hoc network. This way, you can sniff packets from a specific channel without ever needing to transmit any packets. In monitor mode, you can also discover access points that might not otherwise be available to you and attempt to crack the WEP key associated with that AP that way.

AirSnort also allows you to use promiscuous mode, which is similar to monitor mode but requires you to associate with a nearby access point-meaning you can effectively only sniff networks that trust you. If you are in promiscuous mode, you will not be able to sniff packets until after you have associated with an AP. "

http://www.securityfocus.com/infocus/1785