In construction, a firewall consists of a windowless, fireproofwall (or a wall of substantially heavier construction than other walls in a building) built to prevent fire from spreading beyond one section of a building. Such firewalls form the built-up equivalent of firebreaks in a landscape. Their function in containing the undesirable resembles the use of bulkheads in shipbuilding and aircraft construction.
Firewalls are also found in specially prepared cars for competition use. For example, a typical conversion of a production car for rallying will include a metal firewall which seals the fuel tank apart from the interior of the vehicle. In the event of an accident resulting in fuel spillage, the firewall can prevent burning fuel from entering the passenger compartment, where it could cause serious injury or death. Firewalls have to be fitted so that they form a complete seal — usually this is done by bonding the metal sheet to the bodywork using fiberglassresin.
The term firewall is also commonly used by automotive mechanics to refer to the barrier between the passenger and engine compartments of any vehicle.
In computing, a firewall is a piece of hardware and/or software which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the function of firewalls in building construction.
Application-layer firewalls work on the application level of the TCP/IP stack (i.e., all browser traffic, or all telnet or ftp traffic), and may intercept all packets traveling to or from an application.
Firewalls often have network address translation (NAT) functionality, and the hosts protected behind a firewall commonly use so-called "private address space", as defined in RFC 1918.
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