What is a firewall?

The name conjures a solid barrier, but a firewall does not block everything. A computer connected to a network constantly sends and receives data, and most of that traffic is wanted and harmless. A firewall sits at the boundary, between your device or network and the outside world, and inspects this traffic as it passes. Its job is to let the good through and keep the unwanted out, according to rules it has been given.

A firewall works by checking each piece of traffic against a set of rules. The rules describe what is permitted, for example which kinds of connections are allowed and which programs may reach the internet. Traffic that matches an allowed rule passes; traffic that does not is blocked. This lets a firewall stop unexpected attempts to reach your computer from outside, while still letting your everyday browsing and apps work normally. It is essentially a filter standing guard at the door.

Firewalls come in two broad forms. A software firewall runs as a program on a single device, such as the one built into most modern computer operating systems, protecting that one machine. A hardware firewall is a separate device that guards an entire network at its edge, often built into the router that connects a home or office to the internet. Many setups use both, with one protecting the whole network and another protecting each device.

It helps to be clear about a firewall's limits. A firewall controls the flow of network traffic, deciding what may come and go. It is not the same as antivirus software, which scans files already on a device for harmful programs. The two do different jobs and are often used together: the firewall tries to keep unwanted traffic from getting in, while the antivirus deals with harmful software that may slip through by other means, such as a downloaded file.

This article was written using information from Wikipedia.