A firewall is a system that typically sits at some point of connectivity
between a site it protects and the rest of the network, as illustrated
in Figure 8.20. It is usually implemented as an “appliance” or part of a
router, although a “personal firewall” may be implemented on an enduser
machine. Firewall-based security depends on the firewall being the
only connectivity to the site from outside; there should be no way to
bypass the firewall via other gateways, wireless connections, or dial-up
connections. The wall metaphor is somewhat misleading in the context
of networks since a great deal of traffic passes through a firewall. One
way to think of a firewall is that by default it blocks traffic unless that
traffic is specifically allowed to pass through. For example, it might filter
out all incoming messages except those addresses to a particular set of IP
addresses or to particular TCP port numbers.