CACI 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. Screening routers tend to implement a simplistic security policy.
Proxy gateways have a richer set of choices for security policy.
Firewall architectures can operate at the application layer, transport layer or network layer of the OSI model. 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 are the most visible part of an installation to the outside and they are the most attractive target for attack Firewalls are targets for penetrators. Firewalls should be kept small and simple, running only the required software so that if a penetrator does get through, the firewall does not have further tools such as compilers, linkers or loaders to continue the attack. Firewalls are very important tools in protecting an environment connected to a network.