NAT Technology
and Terminology
NAT is the process of altering the IP header of a packet, so
the source local address of the internal host is replaced in the header by real
global addresses. In some cases, the destination address might also be modified.
This swapping process is performed by a NAT device, usually on the network
perimeter. The NAT server then maintains a table of the translations that allows
returning packets to be addressed with the correct internal address. Figure 5-5
is a simple example of NAT where the local hosts (10.0.0.0 private network) are
translated by the NAT device to global addresses in the 192.168.1.0 network.
When an outside host sends a response to 192.168.1.97 or
192.168.1.98, the NAT router checks the current table of network address
translations, and then replaces the destination address with the original inside
source address.
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Note |
Obviously, 192.168.1.0 isn’t a globally routable “real”
network, but we’ll use only private addresses in all examples for two reasons.
The public addresses belong to someone, and the current or future owners might
not agree with their use. Second, inevitably, someone will build a “practice”
lab like the ones in the book and a slim possibility would exist for impacting
the real owners of the address.
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Cisco IOS Software uses the following terms when working with NAT.
While other implementations might change the terms, the concepts remain the
same:
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Inside local address The configured IP
address assigned to a host on the inside network. Quite often, these addresses
are drawn from the RFC 1918 private address pools. They could also be real
addresses officially assigned to some other organization.
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Inside global address The inside global
address is the translated address. This is the IP address the outside world sees
for an inside host. Typically, these addresses are allocated from a pool of real
IP addresses provided by the ISP.
-
Outside local address The IP address of an
outside host as it appears to the inside network. Because these addresses are
only used on the inside network, they might not necessarily be real public
addresses. They can be assigned from the RFC 1918 private address pools.
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Outside global address The configured IP
address assigned to a host in the outside network. This address is a real
address assigned from the globally routable pool.
NAT translations can occur dynamically or statically, and can be
used for a variety of purposes, as described in the following sections.
Sections
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