Turn on the application firewall in OS X:

1. Choose System Preferences from the Apple menu.

2. Click Security or Security & Privacy.

3. Click the Firewall tab.

4. Unlock the pane by clicking the lock in the lower-left corner and enter the administrator username and password.

5. Click "Turn On Firewall" or "Start" to enable the firewall.
6. Click Advanced to customize the firewall configuration.