NAT66 and the subscriber prefix length

Crist Clark Crist.Clark at
Tue Nov 18 19:33:01 UTC 2008

>>> On 11/18/2008 at 11:03 AM, "Tim Durack" <tdurack at> wrote:
> On Fri, Nov 14, 2008 at 2:28 PM, Mikael Abrahamsson <swmike at>wrote:
>> On Fri, 14 Nov 2008, michael.dillon at wrote:
>>  Not long ago, ARIN changed the IPv6 policy so that
>>> residential subscribers could be issued with a /56
>>> instead of the normal /48 assignment. This was done
>>> so that ISPs with large numbers of subscriber sites
>>> would not exhaust their /32 (or larger) allocations
>>> too soon. Since these ISPs are allowed to assign
>>> a /56 to residential subscriber sites, their initial
>>> IPv6 allocation will last a lot longer and they won't
>>> have to apply for an additional allocation while
>>> everyone is getting up to speed with an IPv6 Internet.
>> We returned our /32 for a /25 (with /22 being reserved) and current plan is
>> to hand out /48s to everybody (unless they need even more space, then
>> they'll have to apply).
>> So, doing /56 to end users just because you happen to have a /32 right now
>> sounds like a bad plan, it doesn't take that many hours to get a larger
>> space if you can justify it (which wasn't that hard for us).
>> We received our /32 (as a /35 I think) back in 2000 or so, policy has
>> changed since then, with RIPE it's not that hard to get a much larger space
>> with a long term growth plan. My hope is that we'll make do with this /22
>> space for at least 5-10 years (67 million customer /48s is quite a lot),
>> unless something really big happens, and then we'll just have to get an even
>> larger space.
>> So message should be that /48 to end users is the way to go, and this
>> should suit residential and SME market without any additional administrative
>> overhead depending on customer size.
>> --
>> Mikael Abrahamsson    email: swmike at 
> This raises questions for me: we are a mixed enterprise/campus environment.
> Recently got a /45 assigned, so we have a /48 per site (it was some work to
> convince ARIN that fancy subnetting to make a /46 stretch a little further
> made no sense.)

A /45? I thought all allocations were on nibble borders for
IP6.ARPA considerations.

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