NAT66 and the subscriber prefix length

Tim Durack tdurack at
Tue Nov 18 19:03:11 UTC 2008

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.)

We have also started offering residential Internet to those living on
campus, which has been very popular (no suprise.) If I'm expected to assign
a /48 per residential user, I'm already out of address space. Should I be
requesting a /32? Is it acceptable to carve the /32 up a little for IPv4
style multi-homing?

I'd rather come to terms with this now before I do any meaningful


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