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6381-A2-GB23-10 January 2005 A-1
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Terminology
What is a Firewall?
A firewall is protection between the Internet and your local network. It acts as the
firewall in your car does, protecting the interior of the car from the engine. Your
car's firewall has very small opening that allow desired connections from the
engine into the cabin (gas pedal connection, etc), but if something happens to your
engine, you are protected.
The firewall in the router is very similar. Only the connections that you allow are
passed through the firewall. These connections normally originate from the local
network, such as users web browsing, checking e-mail, downloading files, and
playing games. However, you can allow incoming connections so that you can run
programs like a web server.
What is NAT?
NAT stands for Network Address Translation. Another name for it is Connection
Sharing. What does this mean? Your ISP provides you with a single network
address to access the Internet with. However, you may have several machines on
your local network that want to access the Internet at the same time. The router
provides NAT functionality that converts your local network addresses to the single
network address provided by your ISP. It keeps track of all these connections and
makes sure that the correct information gets to the correct local machine.
Occasionally, there are certain programs that don't work well through NAT. Some
games and other specialty applications have a bit of trouble. The router contains
special functionality to handle the vast majority of these troublesome programs
and games. NAT does cause problems when you want to run a server. See the
DMZ section below.
What is a DMZ?
DMZ really stands for Demilitarized Zone. It is a way of separating part of your
local network so that is more open to the Internet. Suppose that you want to run a
web server, or a game server. Normal servers like these are blocked from working
by the NAT functionality. The solution is to isolate the single local computer into a
DMZ. This makes the single computer look like it is directly on the Internet, and
others can access this machine.