Example for intranet

I need some help. Im running drupal as an intranet and we want it to be accessible from the internet.

Our internal IP's go from 192.168.10.1 - 70.1

What id like to happen is to have all internal users able to view the front page and our quality documents and such. However, when they access the website from outside of work over the internet they should only be able to see a logon screen.

Help?
- Jordan