LinuxFocus: Setting up IP-Masquerading

“To use IP-Masquerading as explained in this article you need at
least one Linux box with a 2.2.x Kernel. This machine is used to
set up the connection to the Internet. Using Linux as your
connection-sharing box doesn’t mean that you have run Linux in your
internal network. In fact, Linux works well with Windows, Macs, and
other flavours of Unix. This connection-sharing box is what we are
concerned about in this article. It connects on one side to the
Internet and on the other side to your private network. The machine
has therefore at least 2 interfaces and also at least 2 IP
addresses. One of the IP addresses is a public IP address which can
be routed in the Internet. This IP address is usually assigned to
you by your Internet Service Provider the very moment you setup
your modem connection (or what ever you use). The other IP
addresses is a private address which you can assign from one of
these ranges:

• 10.0.0.0 – 10.255.255.255
• 172.16.0.0 – 172.31.255.255
• 192.168.0.0 – 192.168.255.255

This article does not explain how to set up your network. I assume
that your private network is already in place and configured.”

“Basically IP-Masquerading translates internal IP addresses
into external IP addresses. This is called network address
translation and Linux does this by using something called
port-numbers. From the outside world, all connections will seem to
be originating from your Linux box. … Sometimes, IP packets
are special in nature and IP-Masquerading may not work for all
applications, but it works in most cases. There are modules for
ICQ, ftp, and quake that need to be inserted in the Kernel in order
for those special applications to run correctly from the internal
network. In general though, anything that uses only the HTTP (web
browsers), telnet, ssh, or smtp (email) will work fine.”

Complete
Story