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IP NAT Address Terminology
(Page 1 of 3)
As its name clearly indicates, IP
Network Address Translation is all about the translation of IP
addresses. When datagrams pass between the private network of an organization
and the public Internet, one or more of the addresses in these datagrams
are changed by the NAT router. This translation means that every transaction
in a NAT environment involves not just a source address and a destination
address, but potentially multiple addresses for each of the source and
In order to make more clear the explanation
of how NAT operates, several special designations have been developed
to refer to the different types of addresses that can be found in an
IP datagram when NAT is used. Unfortunately, the terminology used for
addressing in NAT can be confusing, because it's hard to visualize what
the differences are between the (often similar-sounding) names. However,
without knowing what these addresses mean a proper understanding of
NAT operation is impossible, so we need to start by explaining them.
NAT Address Terms Based on Device Location (Inside/Outside)
The first way that addresses are
differentiated is based on where in the network the device is that the
address refers to:
- Inside Address: Any device on the
organization's private network that is using NAT is said to be on the
inside network. Thus, any address that refers to a device on
the local network in any form is called an inside address.
- Outside Address: The public internetthat
is, everything outside the local networkis considered the outside
network. Any address that refers to a public Internet device is
an outside address.
Key Concept: In NAT, the terms inside and outside are used to identify the location of devices. Inside addresses refer to devices on the organizations private network; outside addresses refer to devices on the public Internet.
NAT Address Terms Based on Datagram Location (Local/Global)
An inside device always has an inside
address; an outside device always has an outside address. However, there
are two different ways of addressing either an inside or an outside
device, depending on in which part of the network the address appears
in a datagram:
- Local Address: This term describes
an address that appears in a datagram on the inside network, whether
it refers to an inside or outside address.
- Global Address: This term describes
an address that appears in a datagram on the outside network, again
whether it refers to an inside or outside address.
Key Concept: In NAT, the terms local and global are used to indicate in what network a particular address appears. Local addresses are used on the organizations private network (whether to refer to an inside device or an outside device); global addresses are used on the public Internet (again, whether referring to an inside or outside device).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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