6. ADSL Router terms
What is a firewall?
A firewall is protection between the Internet and
your local network. It acts similarly to the firewall in
your car, 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 some-
thing happens to your engine, you are protected.
The firewall in the ADSL Router is very similar. Only
the desired connections that you allow are passed
through the firewall. These connections are normally
originating from the local network; such as web
browsing, checking your email, downloading a file,
and playing a game. However, in some cases, 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 for you to access the Internet
through. However, you may have several machines
on your local network that want to access the
Internet at the same time. The ADSL 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 some spe-
cialty applications have a bit of trouble. The ADSL
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 though. When running a server, please
see the DMZ section below.
What is a DMZ?
DMZ really stands for Demilitarized Zone. It is a way
of separating out 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 ser-
vers 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.
Your machine isn't really directly connected to the
Internet, and it really has an internal local network
address. When you provide the servers network
address to others, you must provide the address of
the ADSL Router. The ADSL Router "fakes" the con-
nection to your machine.
You should use the DMZ when you want to run a
server that others will access from the Internet.
Internal programs and servers (like print servers, etc)
should NOT be connected to the DMZ
What is a Gateway?
The Internet is so large that a single network cannot
handle all of the traffic and still deliver a reasonable
level of service. To overcome this limitation, the
network is broken down into smaller segments or
subnets that can deliver good performance for the
stations attached to that segment. This segmenta-
tion solves the problem of supporting a large num-
ber of stations, but introduces the problem of get-
ting traffic from one subnet to another.
To accomplish this, devices called routers or gate-
ways are placed between segments. If a machine
wishes to contact another device on the same seg-
ment, it transmits to that station directly using a
simple discovery technique. If the target station
does not exist on the same segment as the source
station, then the source actually has no idea how to
get to the target.
One of the configuration parameters transmitted to
each network device is its default gateway. This
address is configured by the network administrators
and it informs each personal computer or other net-
work device where to send data if the target station
does not reside on the same subnet as the source.
If your machine can reach all stations on the same
subnet (usually a building or a sector within a buil-
ding), but cannot communicate outside of this area,
it is usually because of an incorrectly configured
default gateway.
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