NAT66 and the subscriber prefix length

Iljitsch van Beijnum iljitsch at
Wed Nov 19 15:46:05 UTC 2008

On 14 nov 2008, at 14:55, Fred Baker wrote:

> Before we get too deeply exercised, let Margaret and I huddle on it.  
> The issue you raised can be trivially solved by adding the checksum  
> offset to a different 16 bits in the address, such as bits 96..127.

Being checksum-equivalent is important so all protocols that use the  
standard checksum keep working without the NAT66 specifically  
supporting those protocols.

The trouble is that in one's complement math 0xFFFF is equivalent to  
0x0000 which means that there is loss of information, so accommodating  
the difference in the lower bits means some nasty corner cases are  
possible, while if it's in the subnet bits you just lose one subnet.

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