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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  IP Network Address Translation (NAT) Protocol

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IP NAT Static and Dynamic Address Mappings
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IP NAT Bidirectional (Two-Way/Inbound) Operation
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IP NAT Unidirectional (Traditional/Outbound) Operation
(Page 2 of 2)

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.

Key Concept: In unidirectional (traditional) NAT, the NAT router translates the source address of an outgoing request from inside local to inside global form. It then transforms the destination address of the response from inside global to inside local. The outside local and outside global addresses are the same in both request and reply.



Previous Topic/Section
IP NAT Static and Dynamic Address Mappings
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Pages in Current Topic/Section
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2
Next Page
IP NAT Bidirectional (Two-Way/Inbound) Operation
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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