Using the Built-in Tiger Firewall

Sharing a Single Internet Connection with Multiple Computers

If you have multiple computers on which you want to be able to access the Internet, and only one convenient connection to the Internet itself, you can now easily configure Mac OS X to share the connection it has to the Internet with other machines connected to it on other interfaces. Yet again, more sophisticated options are available at the command line, but being able to just click a button and turn your machine into an AirPort base station or allow all your local machines to communicate with the Internet through your modem is wonderfully convenient.

Figure 7.19 shows the Internet subpane of the Sharing preferences pane. Clicking the Start button enables a number of things under Mac OS X's hood and makes useful automatic configuration choices based on the rest of your Network preferences. When enabled, Internet connection sharing allows any computers connected to this one to use this machine's primary connection to connect to the Internet. If your primary connection is via AirPort, any machines connected to this one via ethernet can connect to the Internet by using this machine's AirPort connection. If your primary connection is an ethernet connection, your machine becomes an AirPort base station, and shares its connection to others using AirPort, as well as allowing others on the same ethernet network to connect through it. You can configure typical AirPort base station options, such as the name, a password, whether to enable WEP (Wired Equivalent Privacy) encryption, and, if enabled, whether to use 40-bit or 128-bit encryption. If your connection is a modem, your machine becomes an AirPort base station, as well as shares the modem connection through the ethernet.

Figure 7.19. Sharing your network connection has never been easier.

CAUTION

You have limited configuration options here. You can't enable or disable DHCP services for the shared connection, and you can't configure what interfaces you want to allow sharing on or through. DHCP runs by default when you enable Internet connection sharing, and DHCP IP-address service is provided on any interfaces where your machine hasn't made a DHCP connection itself. This can cause problems if you're using a fixed IP address on a network on which there is also a DHCP server providing addresses for transient clients. Enabling connection sharing when using such a network causes your machine to become a DHCP server to the network, and in doing so, to compete with the legitimate DHCP server. This can cause confusing results and possible network errors.