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TCP/IP Guide by Charles M. Kozierok

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IP NAT Unidirectional (Traditional/Outbound) Operation

Now it's time to get down to the nitty gritty of how it works. There are many different flavors of NAT, and four common ones are covered in this chapter. It makes sense to start by looking at the original variety of NAT described in RFC 1631. This is the simplest NAT method, and therefore the easiest one to explain.

NAT was designed to allow hosts on a private network to share public IP addresses in accessing an Internet. Since most hosts are clients that initiate transactions, NAT was designed under the assumption that a client/server request/response communication would begin with a datagram sent from the inside network to the outside. For this reason, this first type of NAT is sometimes ...

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