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. Data that has passed through the firewall is just as exposed as if there was no firewall. 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 exercise only minor control over the content admitted to the inside.
Inaccurate data or malicious code must be controlled inside the perimeter. Summary Firewalls are very important tools in protecting an environment connected to a network. The environment must be viewed as a whole - all possible exposures must be considered. Firewalls must fit into a larger comprehensive security strategy. They alone cannot secure an environment.