IP NAT Unidirectional (Traditional/Outbound) Operation
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Other Functions of the Router in NAT
Also note that even though I am focusing on the changes that the NAT router makes to addresses, it also has to make other changes to the datagram. Changing any field in the IP header means that the IP Header Checksum field will need to be recalculated. UDP and TCP checksums need to be recalculated, and depending on the nature of the data in the datagram, other changes may also be required. I discuss these issues in the topic on NAT compatibility issues.
This simplified example assumes the existence of just one router between the private and public networks. It is possible to have more than one router between these networks. If this configuration is used, however, it is essential that they both use the same translation table. Otherwise, if the request is processed by router R1 but the response received by router R2, R2 won't know how to translate back the destination address on the incoming datagram. Of course, this makes dynamic mapping extremely difficult: routers would have to coordinate their address mappings.
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