- Background and Firewall Basics Before being able to understand a complete discussion of firewalls, it's important to understand the basic principles that make firewalls work. What is a network firewall? A firewall is a system or group of systems that enforces an access control policy between two or more networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the ...