Your Mac has a built-in firewall, but it’s not enabled by default.The Apple firewall checks all incoming traffic and has rules how to decide which incoming traffic is allowed, denied, or when to ask you what to do. Having an active firewall is very important to block attacks. The Apple firewall allows per default incoming traffic for several applications from Apple and for all signed Apps. You can turn this off.It is also possible to go in stealth mode, which means that your Mac does not respond to “ping” anymore.See also the official Apple FAQ: https://support.apple.com/en-us/HT201642Recommendation:It is also important to track outgoing traffic, which is not directly done by the Apple firewall. You will be very surprised what all the Apps you know, communicate with servers you never heard of.We recommend: Little Snitch from Objective Development Software GmbHHow to fix itmacOS 10.11 – 11.0Choose System Preferences from the Apple menu.Click Security or Security & Privacy.Click the Firewall tab.Unlock the pane by clicking the lock in the lower-left corner and enter the administrator username and password.Click “Turn On Firewall” or “Start” to enable the firewall.Click Advanced to customize the firewall configuration.