My New Mac, Snow Leopard Edition, 2nd Edition

Turning the Firewall On (or Off)

If you plan on connecting your Macintosh to the Internet, or even to a local area network, you need a firewall. Just as you would never leave your car door unlocked in a dangerous neighborhood, you should never leave your Macintosh unprotected when it's connected to the Internet.

The only time you might want to turn off your firewall is to install a different firewall instead. Computers only need one firewall. If you try to run two or more firewalls, each firewall will interfere with the other one. So the lesson is simple. Always have a firewall and only use one firewall at a time. For most people, the one firewall they'll use is the built-in firewall that comes with every Macintosh.

To turn the built-in firewall ...