If you’ve been using WiFi for a while – and if you’re reading this we'll assume you have been – you’ve undoubtedly run across the term SSID. It’s the human readable “network name” associated with a WiFi Network – like “Joe’s Coffee” or “LAX Airport Free WiFi” or depending on your apartment building, perhaps a lewd comment directed toward neighbors. This “network name” is known as the Service Set Identifier. It can be up to 32 characters long and may identify either a Basic or Extended Service Set.
The majority of WiFi networks are Basic Service Sets (BSS). That is to say a single access point with multiple connected clients – be it laptops, tablets, gaming consoles or IoT coffee makers. Every station (both clients and AP) in the BSS are identified by a Basic Service Set Identification (BSSID). The BSSID is derived from the access point’s MAC address. Specifically the MAC address of the wireless NIC as the access point may also have an Ethernet Network Interface Controller with its own unique MAC address.
Extended Service Sets are larger WiFi networks whereby multiple access points, each with their own BSSID, all share the same SSID or “network name”. For instance a college or corporate campus may require many access points to cover the entire property. In this case the SSID is called an ESSID for Extended Service Set Identification, which facilitates client roaming.
A wireless client considers any access point with the same SSID to be part of the same network, and may choose to connect to any of the available APs. This forms some of the fundamental basis of the Evil WPA Twin attack.