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6212-A2 and 6382-A1 ADSL Router User’s Guide
6. 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,
which protects 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, but if something happens to your engine, you are protected.
The firewall in the router is very similar. Only the desired connections that you allow are passed through the firewall.
These connections normally originate from the local network; such as when you are browsing the Web, checking your e-
mail, 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 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.
There are certain programs that do not work well through NAT, including some games. 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.
Your machine is not really directly connected to the Internet, and it really has an internal local network address.
When you provide the server’s network address to others, you must provide the address of the router. The router
fakes the connection 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) should not be connected to the DMZ.
What is a router?
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 segmentation solves the problem of supporting a
large number of stations, but introduces the problem of getting traffic from one subnet to another.
To accomplish this, devices called routers or routers are placed between segments. If a machine wishes to contact
another device on the same segment, 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 router. This address is
configured by the network administrators and it informs each personal computer or other network 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 building), but cannot communicate outside of this area, it
is usually because of an incorrectly configured default router.
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