Now that we've covered in depth what NAT is and how it works, let's discuss security. So far, we've only covered firewalls indirectly, mentioning them here and there while discussing NAT. Let's begin with some basic definitions, and later get to how firewalls are similar to, and different from, NAT packages.

What is a firewall? That's a bit of a religious issue, as firewall means different things to different people. The original meaning of firewall was a barrier, often in a structure, designed to take a certain amount of time to burn through during a fire. For example, a building may have some walls or portions of walls that are firewalls, designed to compartmentalize a fire for a certain amount of time, to limit damage. Some people liken firewalls in the electronic security sense to these barriers, saying they are designed to deter intruders for a period of time, and to compartmentalize parts of the network. So, if there is a breach in one portion of a network, the others aren't instantly affected, too.

Other folks will argue that a firewall is features X, Y, and Z, with X, Y, and Z being whatever features they desire in a firewall. Some say that the firewall is the portion of a security architecture that stops traffic. Others say it includes the pieces that allow certain types of traffic.

The folks who participate in these discussions are the philosophers of firewalls. These discussions often take place on mailing...