Firewall

A firewall is, literally, a wall built to prevent fire from spreading. However the usual use of the word is in the context of a computer firewall, which is a piece of hardware or software put on the network to prevent intruders from compromising the network.

Firewalls are sometimes confused with proxies?, which work at the application level. Firewalls operate at the low level of the TCP/IP? protocol stack as packet filters, not allowing packets to pass the firewall unless they meet the rules defined by the firewall administrator.

Firewalls generally fall into two categories, stateful and non-stateful. Stateful firewalls also hold some information on the state of connections (i.e. established or not) as part of their rules (e.g. only hosts inside the firewall can establish connections on a certain port).

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http://www.faqs.org/faqs/firewalls-faq/