Firewalls as Traffic Gatekeepers

A firewall is a device or software that controls which traffic is allowed in or out of a network, acting as a security gatekeeper. It inspects packets against a set of rules and permits or blocks them accordingly. Firewalls are a cornerstone of network defense, sitting at boundaries like the link between your internal network and the internet to keep unwanted traffic out.

A firewall enforces a rule set (also called an access control list). Each rule says whether to permit or deny traffic based on criteria like source and destination IP address, port number, and protocol. The firewall checks traffic against the rules in order and acts on the first match. This lets administrators precisely define what communication is allowed.