Given how ubiquitous Wi-Fi has become, many Mac users never even think about plugging an Ethernet cable into their computers in order to get Internet access. You can connect many computers to a single AirPort base station or another Wi-Fi access point without ever having to worry about running out of cables or connectors for them to plug into.
However, in some circumstances, one computer may have Internet access while other nearby devices don't. For example, consider these scenarios:
Your Mac connects to the Internet via Ethernet. You want to connect another Mac to the Internet too, but you have only a single Ethernet port and no router, hub, gateway, or wireless access point. (This is a common situation when traveling, for example.)
Your Mac connects to the Internet via Wi-Fi, but a friend's PC, which lacks a Wi-Fi card, can't connect.
You use your cell phone as a modem (as described later in this chapter) in a location without other forms of Internet access, and you want to share that connection with a friend.
In these and other similar situations, you can enable a Mac to function as a basic router and even as a Wi-Fi access point, as long as it has some means of connecting to the Internet and an available interface with which another computer can connect. All it requires is selecting the interface with which your Mac currently connects to the Internet, the interface(s) through which you want to permit others to share your connection, and, in some cases, ...