Sharing your Internet connection
If your computer is connected to the Internet, you can share its connection with other computers on your local network.
For example, if your computer is connected to the Internet using a DSL modem and has an AirPort Card installed, you can share the DSL connection with other AirPort-equipped computers.
Open System Preferences, click Sharing, and then click Internet.
Choose a network port from the "Share your connection from" pop-up menu.
Select "Built-in Ethernet" or AirPort depending on which computers will share your Internet connection.
IMPORTANT: If your Internet connection and your local network use the same port (built-in Ethernet, for example), investigate possible side effects before you turn on Internet sharing. In some cases (if you use a cable modem, for example) you might unintentionally affect the network settings of other ISP customers, and your ISP might terminate your service to prevent you from disrupting their network.