NAT Terminology

NAT Terminology
NAT is often associated with the translation of a private IP address into a registered IP address,
although this is not always the case, as you will see. As an example of private-to-registered
translation, a private IP address of 10.12.2.10 might be translated into the registered IP address
of 135.167.12.2. Cisco uses the following terms to distinguish which IP addresses get translated
into which:
Inside local
The
inside local
address is the IP address used by a host on the private side of the
network. In our example, this would be 10.12.2.10.
Inside global
The
inside global
address is the public, often registered IP address into which the
inside local address will be translated. This is typically a globally unique and globally routable
IP address, which hosts on the outside network would use to communicate with the inside host.
In our example, this is 135.167.12.2.
Obviously, all IP addresses are
routable
in the usual definition of the term, which
is in the context of the OSI model. In this section, by
routable
we specifically
mean that the appropriate hosts on the network have a route to this address.
For example, the Internet backbone routers do not know how to get to the 10
addresses because they don’t have a route entry. So we say that address isn’t
globally routable, although it may be locally routable inside your intranet.
Registered
is also sometimes used in place of globally routable.
Outside global
The
outside global
address is the actual IP address of a host that resides on the
outside public network and is usually a globally unique and globally routable IP address. Our
example did not use an outside global address, but one was assumed to exist as a destination
address, which would be known to our transmitting host as long as no inbound translation of
outside addresses is being performed.
Outside local
The
outside local
address is the IP address used to translate an outside global IP
address. This may or may not be a registered IP address, but it must be routable on the inside
of your network. Our example did not use an outside local address, because our assumption
remains that no inbound translation of outside addresses is being performed.
NAT can be broken into two broad types—NAT and PAT. NAT is the one-to-one translation
of IP addresses from an inside local IP address, usually one from the RFC 1918 space, to
an inside global IP address that is unique and routable on the Internet. However, if NAT is being
performed between two private networks, perhaps to overcome duplicated address space, there
would not have to be any registered addresses involved. As you can see, we need terms other
than private and registered to describe where these address spaces have their domain, which is
why we have defined inside, outside, local, and global.
PAT, which is sometimes referred to as
NAPT (Network Address and Port Translation)
, can
be viewed as a many-to-one translation, because it can take multiple inside local IP addresses
and translate them to one inside global IP address.