NAT66 and the subscriber prefix length

Mikael Abrahamsson swmike at
Fri Nov 14 19:28:47 UTC 2008

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

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