1.1.2 Introducing NAT and PAT

1.1.2 Introducing NAT and PAT

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 internetwork devices running specialized NAT software which 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 Internet Network Information Center (InterNIC) or service provider. This address is likely to be an RFC 1918 private address.
Inside global address – A legitimate IP address assigned by the InterNIC 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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