A group of corresponding stations is called a BSS (Basic Service Set). The BSS can be organized in several ways.

• Independent BSS, or ad hoc. The network is only the members of the BSS, they talk between themselves directly, they self-organize, there is not central autority.
• Infrastructure BSS. The BSS is organized around an Access Point which can bridge traffic out the BSS onto a distribution network. Members of the BSS talk to the AP only. You can often understand a domain by answering the question "who will hear a broadcast". A BSS (data link layer) is defined by who will hear a broadcast from the AP (but not by a station which is not an AP, because of the hidden station problem!).
• ESS (Extended Service Set). A bunch of BSS's connected by a distribution network. The distribution network connects the Access Points. WiFi doesn't specify the protocol that builds ESS's.

Since this is ethernet on the air, each transmitter/receiver has a 48 bit MAC consistent with the ethernet address. That is, same address space, OUI's, and so on. An AP is a bridge between wired and unwired ethernet, so it has two interfaces. As the leader of a BSS, it gives names the BSS by the ethernet address of its air interface. This is called the BSSID.

An ESS is given a name, called the SSID (Service Set ID). This is the thing you type into your network configuration to join a WiFi network.

A packet on the air will have three addresses, source, destination and BSSID (access point address, essentially). The AP takes traffic it receives off the air that has its address and drops it onto its wired interface, eliting its own address. That is, on the wired side, only the source and destination addresses are seen. The address of the AP is not used, either its wired or unwired addresses.

When an AP sends a packet into the air, it uses the source and destination address of the packet it is bridging as found, and adds its own wireless address as the BSSID. A wired station sending to a wireless station uses the wireless stations's ethernet address just as if it were a wired station. The AP picks the packet off the wire, carries it across to its wireless interface, inserting its wireless address as the extra, third address, and sends it out to the destination.