802.11 is rather complex, with multiple layers of just about
everything, including network id's.

There are two layers of 802.11 logical subnets.

The lowest layer is called the "BSS", short for "Basic Service Set";
it indicates a set of nodes within RF range of each other who are on
the same subnet.  Each BSS has a "BSS id", which is a 6-byte MAC
address.

BSS's can be created either by a base station when in infrastructure
mode, or by one of the nodes when in ad-hoc mode; in the former case,
the BSS ID is the 802.11 MAC address of the base station; in the
latter case, the BSS ID is randomly created by the node which creates
the BSS, and other nodes joining the ad-hoc net learn the BSS by
listening for 802.11 management frames.

In general, the BSS id does not need to be visible to the end user
except for debugging..

Each BSS is conceptually part of an "Extended Service Set", or ESS;
when base stations are in use, traffic between BSS's can be bridged
together, and multiple BSS's can be part of a single ESS.

The ESS id is an up-to-32-byte character string -- this is what's
visible in ifconfig as the "nwid" or "network id".

The ESS id is sometimes also called the "SSID"; it's contained in the
periodic beacon frames transmitted by BSS creator.  nodes interested
in joining a 802.11 net can select which net to join based on on the
SSID found in the beacons..

> I wonder what exactly is the nwid?  Is it a plaintext "password" that
> anyone can snoop from the airwaves?  

Well, "password" is a bit strong.  It's more like a "subnet name".
Think of it as analagous to the labels people sometimes put on network
drops to indicate which subnet a particular drop is connected to..

> Is it even worth setting this to something other than the default?

Yes, if there are multiple subnets within radio range of each other..

					- Bill