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Firewalls are security devices used to restrict access in communication networks. They prevent computer access between networks (say from the Internet to your corporate network), and only allow access to services which are expressly registered. They also keep logs of all activity, which may be used in investigations.

With the rapid growth in electronic communications - particularly via the Internet - firewalls, and firewall software, are being installed which will allow remote users to access limited parts of the system but restrict further access without satisfying specific identification and authorisation requirements. For example; an organisations' Web site will contain pages which are available to any Internet 'surfer' but other areas will not be accessible without recognition of authorised user status by the system. See Extranet.

Firewall Machine. A dedicated gateway computer with special security precautions on it, used to service outside network, especially Internet, connections and dial-in lines. The idea is to protect a cluster of more loosely administered machines hidden behind it from intrusion. The typical firewall is an inexpensive microprocessor-based Unix machine with no critical data, with modems and public network ports on it, but just one carefully watched connection back to the rest of the cluster. The special precautions may include threat monitoring, call-back, and even a complete iron box which can be keyed to particular incoming IDs or activity patterns.

Firewall Code. The code put in a system (say, a telephone switch) to make sure that the users can't do any damage. Since users always want to be able to do everything but never want to suffer for any mistakes, the construction of a firewall is a question not only of defensive coding but also of interface presentation, so that users don't even get curious about those corners of a system where they can burn themselves.

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