The Elusive Terminology of NAT

The Elusive Terminology of NAT
After swimming through the easily confused terms that NAT brings to the table, you can probably
see how easy it is, for example, to call an address an outside global address when what you
really mean is inside global. We all think of the private address space as being reserved for the
inside network, which is where the “inside” NAT reference gets its name, but many of us erroneously
extend this idea to the addresses themselves, equating all addresses associated with
inside hosts as local. It’s important to understand that the location of the host defines the inside/
outside characteristic, while the original/translated address spaces define the local/global characteristic,
respectively, with each inside host generally having one of each. If the inside network
natively uses private addresses, then the private addresses are the local addresses. If these
addresses get translated to registered addresses on their way out to the public site of the NAT
server, then the registered translation is the global address, but both of them refer to the same
inside
host. It’s important to note that no host will ever be both inside and outside for the same
translation. Which hosts are inside hosts and which ones are outside hosts will be contingent
upon which router interfaces get the
ip nat inside
and
ip nat outside
commands applied to
them. More about those commands later in this chapter.

Let’s talk about the NAT process; that may help clear up any questions you might have from
the preceding definitions. We’ll talk about how NAT processes packets from the inside to the
outside, and then we’ll discuss the reverse.