IPv6 Subnet tool

[Mike Leber]

Joe Abley wrote:
> Moving from a /56 to a /48 is easy; the annoyance is in the other 
> direction. If your numbering scheme for your assigned /48 is of the form
> 
>   xxxx:xxxx:xxxx:SUBNET:<64-bit-EUI>
> 
> (i.e. you're identifying individual /64 prefixes using some 16-bit 
> integer denoted as SUBNET above), that doesn't map nicely into a /56 
> unless you make the conscious decision not to use the top eight bits of 
> the SUBNET value (in which case you've really only got a /56).
> 
> I'm not saying it's a huge deal, but in general it seems like it is 
> non-zero work. This seems like a shame for an address format that was 
> chosen, in part, to make renumbering easier.

So for a customer to be able to do such migrations, they need to use the 
lower bits first.

If the problem you are posing is: "How can I make 8 bits work when I 
started with 16 bits?", the answer is use the lower 8 bits or renumber 
to fit in the 8 bits, and you are presupposing you have 8 bits or less 
worth of resource to migrate.  It's not like math did you wrong.

This would be a reasonable planning thing to mention to customers with a 
mix of /48s and /56s from various providers.

Mike.

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