A firewall is a kind of valve: it's easy to get out (on the internet), but hard to get in.
       The firewall is one or more programs on a computer, that's also a node between intranet and internet. All packages going through are checked. Some of the packages are discarded. A typical rule for checking is to check which porst are allowed.
       A rule of thumb: a firewall starts as closed, and has to be opened to allow the necessary access.

Concept last updated: 15/09 2003.


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Other sources

Filsystem Firewall Flash