NAT inside/outside and local/global relationship

FIGURE 3 . 1
NAT inside/outside and local/global relationship
The Advantages of NAT
There are many advantages to using NAT. In this section, you will learn about some of the more
important benefits, including the following:

NAT allows you to incrementally increase or decrease the number of registered IP addresses
without changing devices (hosts, switches, routers, and so on) in the network. You still need
to change the device doing the NAT but not every other device.
Which Camp Are You From?
To further the NAT terminology debate, let’s cover an issue that has the NAT-speaking world
firmly divided into two different camps, with most campers completely unaware that there’s
another camp! While it is not technically inaccurate, on a basic level, to consider the outside
local and outside global addresses to be the same when translation of the outside address
space is not being performed, such a habit generally serves only to muddy the waters. Until
you truly have a grasp on the terminology, stick with the more distilled concepts outlined here.
Your ability to keep these terms in their proper context will benefit, and you won’t miss any
questions along the way as a result. Furthermore, one camp maintains that it is simply wrong
to make reference to an outside local address when the outside global address has not been
translated. If you take the basic definition of an outside local address, you’ll find that the outside
local address space, indeed any local address space, must be routable on the inside network.
With that basic tenet in mind, calling the outside global address—which, as a global address,
must be routable on the outside network, not necessarily on the inside network—an outside
local address simply makes no sense. As mentioned earlier, it also muddies the waters. Does
this remind you of high school geometry proofs? Do yourself a favor. Because the converse
cannot be proven quite so easily, run as fast as you can to the camp that believes the outside
global address—by definition, an address of a node on the outside network that is routable on
the outside network—can never be called a local address of any kind.
Translation
Direction
Translation
Direction
NAT
Inside Network Outside Network
Inside
Host
Outside
Host
SA Inside
Global
DA Outside
Global
DA Inside
Global
SA Outside
Global
SA Inside
Local
DA Outside
Global
DA Inside
Local
SA Outside
Global