When we talk about "wireless LAN," we are generally referring to the link and physical layers of the network. The IEEE 802 standards deal with these layers for a range of different LAN technologies, including IEEE 802.3, which is commonly (but incorrectly) called "Ethernet" [IEEE 802.3]. IEEE 802.11, of course, is the very widely deployed standard for Wi-Fi wireless LAN. In most LAN technologies, the same type of LAN must exist at both ends of a link. In other words, an Ethernet cable connects an Ethernet port on a computer to an Ethernet port on a hub. The equivalent to the LAN hub in IEEE 802.11 is the access point, which acts like the center of a wheel in distributing data for most Wi-Fi LANs. When IEEE 802.11 systems work through an access point, they are said to be operating in infrastructure mode because the access point is coordinating the Wi-Fi LAN from a fixed point and often providing a connection to a wired Ethernet network.
In the early days of Ethernet, you could use a hub (like all systems today) or you could connect all the computers together using a single coaxial cable. In the latter case, you didn't need a hub because the single cable joined all the computers together in series. When any computer sent a message, all the others could potentially receive it, but only the recipient to whom it was addressed would actually listen. IEEE 802.11 has a similar mode called ad-hoc mode. In this case no access point is needed and each wireless device can transmit directly to any other. It was intended to be useful for groups of people who wanted to set up a network anywhere and share information hence, "ad-hoc."
To summarize, IEEE 802.11 has two modes, infrastructure and ad-hoc, sometimes referred to by the technical acronyms ESS and IBSS, respectively. From a security standpoint, ad-hoc networks present quite a challenge and we will deal with them separately in a later chapter. Most people operate in infrastructure mode because they want to be able to connect to a wired infrastructure such as a local Ethernet or an Internet connection. Infrastructure mode also offers a much better platform for building security. Most of what is described in the following chapters refers to operation in infrastructure mode.