Mastering™ Network Security, Second Edition

5.2. Definition of a Firewall

A firewall (unlike a simple router that merely directs network traffic) is a system or group of systems that enforces an access control policy on network traffic as it passes through access points. Once you determine the levels of connectivity you want to provide, it is the firewall's job to ensure that no additional access beyond this scope is allowed. It is up to your firewall to ensure that your access control policy is followed by all users on the network.

Firewalls are similar to other network devices in that their purpose is to control the flow of traffic. Unlike other network devices, however, a firewall must control this traffic while taking into account that not all the packets of data it sees are what ...