A firewall is an electronic barrier that prevents unauthorized access to a network. The role of a firewall is to ensure that all communications between a network and the Internet, in both directions, conform to the criteria established by the organization's security policies. The network may be a company's local area network, wider area networks, and the Internet, or a Service Provider containing confidential files for which authorized admission is required.

The purpose of a firewall is to protect a network from unwanted or hostile intrusion. To accomplish this task, a firewall functions as a gateway between the internal private network and the unregulated public network.

The name firewall arose from the fact that they function by segmenting a network into several subnetworks in order to limit the damage that might otherwise spread unhindered through the network. By analogy with the firedoors or firewalls in buildings, the term...