The cheksum is a 16 bit one's complement sum of all 16 bit words in the IP
header. To compute the new checksum after translation, the old IP address is
substracted from the checksum and the new one is added to it. Since IP
address is 4 byte quantity, the addition and substraction are done in 2
steps, using 2 bytes in each step. The addition and substraction must be in
1's complement form.I hope i have confused u enough.
For details on checksum computation, please refer to
http://www.faqs.org/rfcs/rfc1071.html
also try rfc1624 and rfc1141.
Regards
Jayant
----- Original Message -----
From:
To:
Sent: Wednesday, June 06, 2001 10:17 AM
Subject: NAT [7:7348]
> Hi All
> I just have a question regarding NAT
> In Network Address Translation (NAT), one ip address has to be
> substituted with another. When this happens, the ip-layer
> checksum must be recalculated. What is the quickest way possible
> to do this operation, and give your own flavour of pseudo (or real
> if you want) code to show how it is done.
> Any sugestions would be greatly appriciated
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