A system designed to prevent unauthorized access to or from a network; also known as a "drawbridge". Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private or restricted networks, especially intranets. All messages passing through the firewall are examined, and those which do not meet the specified security criteria are blocked. There are several types of firewall architecture: screened-host or packet filter, application gateway, circuit-level gateway, dual-host or proxy server. In practice, many firewalls use two or more of these techniques in concert. A firewall is considered to be the initial defense in protecting privileged data or sensitive information. For greater security, data can be encrypted. See password, PGP, SSL, proxy, trap door, escrow key, virus, malware, software.