1. Introduction

This is intentionally short and to the point.

If you have a network, that you want to attach to the outside:

   _____________
  /             \ external IP _________   internal IP
 |   Internet    | 123.12.23.43   | linux   |  192.168.0.1
 |   google.com  | <----[outside card]-| machine |-[internal card]
  \_____________/                       ---------          |
                                                           |
                          _____________              ______|_______
                         | 192.168.0.2 |            /              \
                         |  internal   |-----------|  192.168.0.0   |
                         |  machine    |           |    Intranet    |
                          -------------             \______________/