Mac OS X includes a built-in firewall, which you can enable on the Services tab of the Sharing preference panel. When the firewall is on, communication to any service not allowed in the list is blocked. (In firewall-speak, these entries are called rules because they determine what's allowed to pass through to your Mac.)

To enable communications with a service, select the entry in the list and enable the On check box. Click the New button to specify a new rule by entering a port and assigning it a name. After you create a new rule, it appears in the list, and you can toggle it on and off like any of the default rules. Click the Edit button to edit the selected rule; click the Delete button to remove the selected rule.

If you're using an Internet or network connection, you should enable the firewall to protect your Mac. However, if you suddenly can't connect to other computers or share files that you were originally able to share, it's time to review the rules that you've enabled on this panel.