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 |
------------- \______________/