This is a general introduction to the concepts of firewalls, why they are needed and what they can and cannot do. We will reference terms defined in this book, from time to time. Security, in its most general form, is the combination of all mechanisms whereby you allow only authorized people to use the resources they are supposed to. In the traditional physical world, this involves locks, alarms, and guards. Of course there are people who can pick locks or go through the windows, so you can either buy a better lock, buy an alarm system, or hire a guard.
Computer security is mostly the same.
On concept which is useful to both computer security is that of perimeter security . They typically only have a small number of entrances which are the only way in. If someone wants admittance, they must present themselves at the gate and identify themselves to the satisfaction of the guards. If someone tries to get in any other way, they will be stopped by the various fences, walls, or barriers surrounding the installation. If you can insure that those are the ONLY ways in to the internal network, then if you can sufficiently guard those small numbers of access points, you can guard all of the machines on the network.