How to Make Your Mac a WiFi Access Point
If you have a WiFi-equipped Mac connected to the Internet by a wired Ethernet connection or even a dialup modem, you can turn that Mac into a WiFi access point and share the connection with other WiFi-equipped Macs and even PCs.
The heart of a WiFi network is an access point that supervises the communications. An access point may be built into your Ethernet router, or it may be a stand-alone unit. Each access point has a limited range, up to about 300 feet, or 100 meters. However, you can have more than one access point, and they can talk to each other. To let WiFi computers within range access the Internet, at least one access point has to connect to your Internet modem, usually with an Ethernet cable.
To make your Mac a WiFi access point, follow these steps on the computer that has the Internet connection: Select System Preferences from the Apple menu, click the Sharing icon, and then click the Internet button. From the Share Your Connection From popup menu, select the way this computer is connected to the Internet: typically Ethernet, Modem, or Bluetooth.
In the To Computers Using section, select the AirPort check box. Click the AirPort Options button and assign your new network a name. Turn on encryption if you like, enter a password, and click OK. Back in the Sharing pane, click the Start button in the Internet Sharing section.
Your other computers should now see your new WiFi network. When you no longer want your Mac to serve as an access point, go back to the System Preferences Sharing pane and click the Stop button in the Internet Sharing section.