What is a firewall?

A (Internet) firewall is a machine which is attached (usually) between your site and a wide area network. It provides controllable filtering of network traffic, allowing restricted access to certain internet port numbers (ie: services that your machine would otherwise provide to the network as a whole) and blocks access to pretty well everything else. Similar machines are available for other network types, too.

Firewalls are an effective "all-or-nothing" approach to dealing with external access security, and they are becoming very popular, with the rise in Internet connectivity.

For more information on these sort of topics, see the Gateway paper by [Cheswick], below.