The firewall is normally an intermediate system between the secure internal networks and the less secure external networks. It is intended to keep corporate systems safe from intruders, hackers, and accidental entry into the corporate system. The primary types of firewalls are screening routers, proxy servers, and stateful inspectors. Before choosing a firewall architecture, a company must have the right mind set regarding threat. The purpose of this paper is to provide an introduction to firewall concepts and help develop this mind set.