Page 184 - CISSO_Prep_ Guide

Quite often, the IP address provided is based on the addresses
currently used for internal addresses - known as the non-routable
addresses - in RFC 1918.

RFC 1918 lists four addresses spaces that are non-routable and
are most often used for internal addressing:
The entire 10.x.x.x network (where x is any value from 0 - 255)
The range of 172.16.x.x to 172.31.x.x
The 192.168.x.x network

This process is known as Network Address Translation (NAT).
The internal addresses used on the network are for internal use
only and could not be routed over the internet. This is similar to
having an internal mail address for an office tower. Each cubicle
or desk has an internal address (often comprised of the floor in
the building and area on the floor). The mail to the company
comes to an external address - perhaps a box number or street
address. A clerk in the mailroom then sorts the mail and routes it
to the internal addresses. A NAT firewall does the same thing.
When traffic is going out from the internal network, the firewall
changes the source address (the address of the person going out
of the network) to the external address of the firewall. This
allows the remote device that the user wants to talk to, to be able
to reply by sending its reply to the external address of the
firewall. The firewall then converts the destination address of
the reply to the internal address of the party that requested the
information.