A firewall protects your computer from Internet worms that exploit open ports and operating system vulnerabilities. Follow these instructions to activate the firewall on your Mac OS 10.6 (Snow Leopard) computer.

1. From the Apple menu, click System Preferences.
2. In the System Preferences window, click the Security icon in the top row, highlighted in the following graphic:
   
3. In the Security window, click the Firewall tab.
4. If not already enabled, click Start to turn on the firewall. The window should now look similar to the following:
   
5. Click Advanced in the lower-right corner of the window.
6. Check the Block all incoming connections check box if it's not already checked. This should also automatically check Enable stealth mode near the bottom of the window. The window should now look similar to the following:
   
7. Click OK.
8. Close the Security window.