A firewall creates a virtual barricade between one part of a network and another, preventing all kinds of data from passing in and out, depending on how the firewall is configured. A firewall can protect an entire network, but more typically, you install a firewall on a single computer to prevent other computers on a local network or the Internet from accessing any services that you haven't specifically allowed.
To turn on Leopard's built-in firewall service, open the Firewall view of the Security system preference pane. You have three distinct options to choose among:
Allow All Incoming Connections: This default option blocks no traffic. The firewall is off.
Block All Incoming Connections: An extreme option, this ...