In traditional wired networks, those responsible for the existence of the network can exert a high degree of control over what happens on their wires. Through border firewalls, proxies, packet filters, and clever routing, the ultimate network content available to an individual node can be manipulated to an almost infinite degree.
The rules are very different when the wires are taken away. Anyone with an 802.11b card can effectively generate whatever sort of packet they like and send it out to anyone within range. As long as nodes can agree on a common method of communications, any number of peer-topeer networks can be created to exchange data, in a way that makes it prohibitively difficult for a single entity to impose any sort of restriction on the flow of that data.
IBSS mode is one of the most liberating aspects of the 802.11b protocol. It effectively makes expensive access point hardware entirely optional, relying instead on each node of the network to maintain its own communications. Instead of a centralized model, where all clients must be within range of an access point in order to participate, IBSS allows any node to talk to any other node within earshot. If one of those nodes happens to be a gateway to the Internet, it can not only provide access-point-like services but also do the sorts of things that any self-respecting gateway can do: route packets, throttle bandwidth, act as a firewall, etc.
While IBSS refers to a specific mode of 802.11b operations, you will also encounter a couple of other similar (but loaded) terms in your travels: ad-hoc and peer-to-peer. A few vendors have implemented their own "AP-less" mode of operations that, while occasionally providing some interesting features, don't conform to the IEEE standard and will work only with their own hardware. These proprietary modes are referred to in product literature as ad-hoc or peerto-peer mode. Cards that claim 802.11b compliance may support these additional proprietary modes, but they must also support true IBSS communications.
To further compound the confusion, the Linux wireless tools package calls any non-AP mode ad-hoc and lets you choose which one it uses (IBSS or proprietary) via a system call. In the interest of maximum compatibility, we will be dealing only with true IBSS mode for the rest of the book, and we'll use the terms IBSS, ad-hoc, and peer-to-peer interchangeably (unless otherwise specified).