Linux.com: IP Masquerading on your Network with Red Hat Linux

Using your Linux box to share a single internet connection
is one of the most popular ways to use an old beat up 486 or
Pentium machine that otherwise would have been thrown into the junk
pile in the garage.
When used in conjunction with a fast
Ethernet connection like cable, or DSL. Using Linux as your
connection-sharing box doesn’t mean that you have to only run Linux
on your internal network (your office or your home). In fact, Linux
will work well with Windows, Macs, and other flavors of Unix.”

“Throughout this article, I will be assuming you have some
networking experience (using Linux or any other operating system),
and are familiar with the concepts of a firewall, network, and IP
addresses. For compatibility with this article you should be
running a recent version of Red Hat Linux, SuSE Linux, or Mandrake
Linux. You should be comfortable with entering commands on the
Linux command line.”

“The minimum requirements are one Linux box, and one machine on
the internal network. We will be using Red Hat Linux throughout
this tutorial. The basic idea is to connect your Linux box to the
internet, then have all other machines on your internal network
talk to the Linux box if they need something from the general
internet. In order to make it as secure as possible, you want to
have two network cards in your Linux box, and at least one network
card on the machine that you want to use internally.”

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