The humble firewall has saved many a network. And yet the premise as to how it works is actually very simple: let the good stuff in, block the bad stuff. Shown below, at the most basic level, is a firewall at work. Hint: It acts as a wall, as the name suggests.
 

A firewall can be either a piece of hardware or software, and its gatekeeping can be likened to a 24/7 shield for your private network. It can also be likened to a “super” security guard—one who knows thousands, if not millions, of criminals, and uses this encyclopaedic knowledge to determine who can and cannot enter.

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