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IP Network Address Translation (NAT) Protocol
To help extend the life of the IPv4 addressing
scheme while the newer IPv6
protocol is developed and deployed, other
technologies have been developed. One of the most important of these
is IP Network Address Translation. This technology allows a small
number of public IP addresses to be shared by a large number of hosts
using private addresses. This essential little trick allows
the global Internet to actually have far more hosts on it than its address
space would normally support. At the same time, it provides some security
benefits by making hosts more difficult to address directly by foreign
machines on the public Internet.
In this section I provide a description
of the concepts behind IP NAT and an explanation of operation of IP
NAT types. I begin with an overview of the protocol and discussion of
its advantages and disadvantages. I describe the address terminology
that you need to know to understand how NAT functions and the differences
between various translation techniques. I explain the way that address
mappings are performed and the difference between static and dynamic
I then explain the operation of the
four main types of NAT:
- Unidirectional NAT (also called
outbound or traditional NAT)
- Bidirectional (inbound or two-way)
- Port-Based or Overloaded
NAT (also called NAPT or PAT)
- Overlapping NAT
(also called Twice NAT).
I conclude with a bit more information
on compatibility issues associated with NAT.
Incidentally, most people just call
this technology Network Address Translation without the
IP. However, this sounds to me rather generic, and since
the version being discussed here is specific to IP, I prefer to make
it clear that this is an IP feature. That said, for simplicity I often
just say NAT too, since that's shorter. I should also point
out that there are quite a few people who dont consider NAT to
be a protocol in the strictest sense of the word.
Note: NAT was developed in large part to deal with the address shortage problem in IPv4, so it is associated and used with IPv4. It is possible to implement an IPv6-compatible version of NAT, but address translation isn't nearly as important in IPv6, which was designed to give every TCP/IP device its own unique address. For this reason I focus in this section on the use of NAT with IPv4.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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