Before continuing with this chapter, you should be familiar with the following Cisco terms:
Inside network
The
inside network
is the set of network addresses that is subject to translation.
The IP addresses used within the network are invalid on an outside network such as the
Internet or the network’s ISP. Often, the IP addresses used in the inside network are obsolete,
or an IP address is allocated in a range specified by RFC 1918 or RFC 3330 (which reserves certain
IP addresses for internal use only) and is not Internet routable.
Outside network
The
outside network
is not affiliated with or owned by the inside network
organization. (Keep in mind we are referring to a network—not network addresses.) This can
be the network of another company when two companies merge, but typically is the network
of an ISP. The addresses used on this network are legally registered and Internet-routable
IP addresses.
Inside local IP address
The
inside local IP address
is the IP address assigned to an interface in
the inside network. This address can be illegal to use on the Internet, or it can be an address
defined by RFC 1918 as unusable on the Internet. In both cases, this address is not globally
routable. If the address is globally routable, it can be assigned to another organization and cannot
be used on the Internet.
Inside global IP address
The
inside global IP address
is the IP address of an inside host as it
appears to the outside network. This is the “translated IP address.” Addresses can be allocated
from a globally unique address space, typically provided by the ISP (if the enterprise is connected
to the global Internet).
Outside local IP address
The
outside local IP address
is the IP address of an outside host as it
appears to the inside network. These addresses can be allocated from the RFC 1918 space if desired.
Outside global IP address
The
outside global IP address
is the configured IP address assigned
to a host in the outside network.
Simple translation entry
A
simple translation entry
is an entry in the NAT table that results
when the NAT router matches an illegal inside IP address to a globally routable IP address that
is legally registered for Internet use.
Extended translation entry
An
extended translation entry
is a translation entry that maps one
IP address and port pair to another.