Firewall

A firewall separates two networks from each other, typically an internal company network and the internet, and decides based on a fixed rule set which connections it lets through. Without this control point, every device on the network would be directly reachable from the internet. The underlying idea is to block everything by default and only open what is actually needed.

In practice, a firewall is usually a dedicated device sitting between the internet connection and the internal network, though it is often built directly into the router supplied by the internet provider. It works from rules that define which ports and which source or destination addresses may pass through. A firewall that is working correctly barely shows up in daily operations; it usually only becomes visible when a new application needs an additional rule opened.

A common misunderstanding is that a firewall also controls traffic moving between devices inside the same network – it does not, that requires network segmentation on top of it. It is also common for the firewall built into the provider's router to be left on its factory default, with rules nobody has reviewed since installation. A firewall with an unmaintained rule set offers barely more protection than none at all.