Mobiwol

Mobiwol is the only firewall for Android devices that will work without you needing to root your smartphone. In other words, all you have to do is download the app and, without having to mess with any complicated operations, you'll have your device secured and protected.

From the application's options you can control all of the configurations of the rest of the apps you have installed, easily blocking their Internet connectivity so that they won't access the Internet without your permission (something that a lot of them do). You can also choose to receive alerts each time a new application tries to access the Internet.

Although Mobiwol runs automatically each time you start up your device, you can change this, too. You can use it only when you want to check the connection of a certain app, and have it turned off the rest of the time by default.

Mobiwol is an excellent firewall tool for Android, as it gives your device an additional layer of protection without needing to root the device to do so.