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NAT is designed to conserve IP addresses and enable
networks to use private IP addresses on internal networks. These
private, internal addresses are translated to routable, public
addresses. This is accomplished by inter-network devices running
specialized NAT software and can increase network privacy by hiding
internal IP addresses.
A NAT enabled device
typically operates at the border of a stub network. A stub network is
a network that has a single connection to its neighbor network.
 When
a host inside the stub network wants to transmit to a host on the
outside, it forwards the packet to the border gateway router. The
border gateway router performs the NAT process, translating the
internal private address of a host to a public, external routable
address.
 In NAT terminology, the internal network is the set
of networks that are subject to translation. The external network
refers to all other addresses.
Cisco defines the following NAT terms:
- Inside local address – The IP
address assigned to a host on the inside network. The address is
usually not an IP address assigned by the Network Information Center
(NIC) or service provider. This address is likely to be an RFC 1918
private address.
- Inside global address – A
legitimate IP address assigned by the NIC or service provider that
represents one or more inside local IP addresses to the outside
world.
- Outside local address – The
IP address of an outside host as it is known to the hosts on the inside
network.
- Outside global address – The
IP address assigned to a host on the outside network. The owner of
the host assigns this address.
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Interactive Media Activity
Drag and Drop: Basic Network Address Translation
When the student has completed this activity, the student
will be able to identify the IP address translations that occur
when using NAT.
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