In this presentation, you will find out how to set up on-demand-dialup to the internet using PPP, how to configure your Linux box to act as a gateway to the internet with only one IP address using IP Masquerading, and finally how to set up services that are behind the gateway to be reachable from the outside.
While it is not necessary to set up all of these, they are all related. If you just want to use demand dialing, no steps other than the second one are required.
About IP MasqueradingSeveral steps are necessary, including:
Presented at the St. Louis Unix Users Group Linux SIG, 21 August, 1997 by Jerry Adlersfluegel, jerry@dar.net