A Primer A firewall is a process that filters all traffic between a protected or inside network and a less trustworthy or outside network. Firewalls implement a security policy. A firewall is a gateway which all traffic passes through to and from a protected network. Proxy gateways have a richer set of choices for security policy. A guard is a sophisticated proxy firewall.
Guards decide what services to perform on a users behalf.
Firewall architectures can operate at the application layer, transport layer or network layer of the OSI model. Firewalls are not complete solutions to all computer security problems. They protect the perimeter of the environment against attacks. If one inside host connects to an outside address by a modem the entire inside net is vulnerable through the modem and its host. Firewalls do not protect data outside the perimeter.