NAT is used in the following situations:
■ When you want to connect to the Internet, but not all hosts have globally unique IP
addresses: NAT technology enables private IP internetworks that use nonregistered IP addresses to connect to the Internet. A NAT router is placed on the border of a stub domain (referred to as the inside network) and a public network, such as the Internet (referred to as the outside network). The NAT router translates the internal local addresses into globally unique IP addresses before sending packets to the outside network.
NAT takes advantage of the fact that relatively few hosts in a stub domain communicate outside of the domain at any given time. As a result, only a subset of the IP addresses in a stub domain must be translated into globally unique IP addresses when outside communication is necessary.
■ When you need to modify your internal addresses because you are changing Internet service providers (ISP): NAT can be used to translate the appropriate addresses. This enables you to change addresses incrementally, without changing hosts or routers other than those that border stub domains.
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