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IP NAT Address Terminology
(Page 2 of 3)
Combining Inside/Outside and Local/Global Address Designations
This is a bit confusing, so I will
try to explain further. The NAT translating router has the job of interfacing
the inside network to the outside network (the Internet). Inside devices
need to be able to talk to outside devices and vice-versa, but inside
devices can only use addressing consistent with the local network addressing
scheme. Similarly, outside devices cannot use local addressing. Thus,
both inside and outside devices can be referred to with local or global
address versions. This yields four different specific address types:
- Inside Local Address:
An address of a device on the local network, expressed using its normal
local device representation. So for example, if we had a client on a
network using the 10.0.0.0 private address block, and assigned it address
10.0.0.207, this would be its inside local address.
- Inside Global Address:
This is a global, publicly-routable IP address used to represent an
inside device to the outside world. In a NAT configuration, inside
global addresses are those real IP addresses assigned
to an organization for use by the NAT router.
Let's say that device 10.0.0.207
wants to send an HTTP request to an Internet server located at address
126.96.36.199. It forms the datagram using 10.0.0.207 as the source address.
However, if this datagram is sent out to the Internet as is, the server
cannot reply back because 10.0.0.207 is not a publicly-routable IP address.
So the NAT router will translate 10.0.0.207 in the datagram into one
of the organization's registered IP addresses, say it's 188.8.131.52.
This is the inside global address that corresponds to 10.0.0.207.
It will be used as the destination when the server sends its HTTP response.
Note that in some situations the inside local address and outside local
address may be the same.
Local/Global Address Designations from the Perspective of Device Location
- Outside Global Address:
An address of an external (public Internet) device as it is referred
to on the global Internet. This is basically a regular, publicly-registered
address of a device on the Internet. In the example above, 184.108.40.206
is an outside global address of a public server.
- Outside Local Address:
An address of an external device as it is referred to by devices on
the local network. In some situations, this may be identical to the
outside global address of that outside device.
Phew, it's still confusing, isn't
it? Let's try another way of looking at this. Of these four addresses,
two types are the addresses as they are known natively by
either an inside or outside device, while two are translated addresses:
- Inside Device Designations: For
an inside device, the inside local address is its normal
or native address. The inside global address is a
translated address used to represent the inside device on the outside
network, when necessary.
- Outside Device Designations: For
an outside device, the outside global address is its normal/native
address. The outside local address is a translated address used
to represent the outside device on the inside network, when necessary.
So, what NAT does then is translate
the identity of either inside or outside devices from local representations
to global representations and vice-versa. Which addresses are changed,
and how, depends on the specific type of NAT employed. For example,
in traditional NAT, inside devices refer to outside devices using their
proper (global) representation, so the outside global and outside local
addresses of these outside devices are the same.
Key Concept: A NAT router translates local addresses to global ones, and vice-versa. Thus, an inside local address is translated to an inside global address (and vice-versa) and an outside local address is translated to an outside global address (and vice-versa).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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