Internet and computer basics

Firewall

A system or device at the entrance of a network that lets through only the traffic its rules allow and blocks the rest, like a reception desk that only lets in expected visitors. It can be built into a router or be a separate device or program. A UTM (unified threat management) device combines a firewall with other protections such as virus checks; a WAF (web application firewall) watches only for attacks on websites. A gap in the rules, or a vulnerability in the firewall itself, lets attackers through.