Firewall

A firewall's basic task is to regulate the flow of traffic between computer networks of different trust levels. Typical examples are the Internet, which is a zone with no trust, and an internal school or school system network, which is a zone of higher trust.  A firewall's function within a network is similar to firewalls with fire door in building construction. In former case, it is used to prevent network intrusion to the private network. In latter case, it is intended to contain and delay structural fire from spreading to adjacent structures.

Without proper configuration, a firewall can often become worthless. Standard security practices dictate a "default-deny" firewall ruleset, in which the only network connections which are allowed are the ones that have been explicitly allowed. Unfortunately, such a configuration requires detailed understanding of the network applications and endpoints required for the organization's day-to-day operation. Many schools lack such understanding, and therefore implement a "default-allow" ruleset, in which all traffic is allowed unless it has been specifically blocked. This configuration makes inadvertent network connections and system compromise much more likely.

Some text in this entry is taken from Wikipedia's firewall page, and is licensed under the terms of the GNU Free Documentation license.