A firewall serves as a barrier between a LAN and the Internet. It allows private resources to remain private while reducing security threats. It manages both inbound and outbound network traffic.

A sample firewall between a LAN and the internet is shown in the diagram below. The point of vulnerability is the connection between the two. At this point, network traffic can be filtered using both hardware and software.

There are two types of firewall systems: one that uses network layer filters and the other that uses user, application, or network layer proxy servers.