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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.
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.
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Version 3.0 - Version Date: September 20, 2005
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