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From a NAT perspective, an inside network is the local portion of a network that uses
private, not publicly routable IP addresses that you want to translate. An outside
network is the public portion of a network that uses legitimate, publicly routable IP
addresses to which you want private hosts to connect.
The addresses that are translated by NAT between address realms are labeled as
inside or outside, and as local or global. When reading the terms in the following
sections, keep the following definitions in mind:
Inside Local Addresses
The inside local address is a configured IP address that is assigned to a host on the
inside network. Addresses may be globally unique (not requiring translation), allocated
from the private address space defined in RFC 1918, or officially allocated to some
other organization.
Inside Global Addresses
The inside global address is the translated IP address of an inside host as seen by an
outside host and network. Addresses may be allocated from a globally unique address
space (often provided by the ISP, if the inside address is connected to the global
Internet).
Outside Local Addresses
The outside local address is the translated IP address of an outside host as it appears
to the inside network. Addresses may be globally unique (not requiring translation),
allocated from the private address space defined in RFC 1918, or officially allocated
to some other organization.
Outside Global Addresses
The outside global address is the configured, publicly routable IP address assigned to
a host on the outside network.
Understanding Address Translation
Address translation can occur one of two ways: inside or outside source translation.
Inside Source Translation
Inside source translation is the most commonly used NAT configuration. When an
inside host sends a packet to the outside network, the NAT router translates the
source information (either the source address or the source address/port pair) and,
in the inbound direction, restores the original information (this time operating on
the destination address or address/port pair).
The terms inside and outside refer to the host that the address is associated with.
The terms local and global refer to the network on which the address appears.
Chapter 2: Configuring NAT
Understanding Address Translation
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