IPv6 End User Assignments

michael.dillon at bt.com michael.dillon at bt.com
Sat May 9 14:23:49 CEST 2009

I just posted the following to a private ARIN members'
mailing list, and I thought I would post it here to
see if there are any comments on the advice that I gave.

> > /48 as per standard.
> What standard are you referring to?

"As per standard" is a colloquial way of saying "standard practice"
which refers to the way that people have been doing things for the past
few years. Of course it all started with an RFC like 3177 which says:

-----quoted text------
This document provides recommendations to the addressing registries
(APNIC, ARIN and RIPE-NCC) on policies for assigning IPv6 address blocks
to end sites. In particular, it recommends the assignment of 

   /48 in the general case, /64 when it is known that one and only one
   subnet is needed and /128 when it is absolutely known that one and
   only one device is connecting.

The original recommendations were made in an IAB/IESG statement mailed
to the registries on September 1, 2000. This document refines the
original recommendation and documents it for the historical record. 
-----end quote-------

> From what I'm reading, a longer prefix, such as /56 is acceptable:

Yes it is acceptable but it is not standard practice. That change to
policy was adopted to address an issue that a few very large ISPs have
with the HD ratio.

The change was introduced with policy proposal 2005-8 but please note
the first sentence of the policy text of these guidelines:

The following guidelines may be useful
(but they are only guidelines):
- /64 when it is known that one and only
  one subnet is needed
- /56 for small sites, those expected to
  need only a few subnets over the next 5 years.
- /48 for larger sites

A large part of the rationale for this change to allow /56 was in an
earlier version of this Internet draft:

There is a good section there explaining how /56 for residential users
still maintains RFC 3177's goals.

> "Ideally, residential networks would be given an address range of a
> /48 or /56 [RIPE_Nov07] such that multiple /64 subnets could be used 
> within the residence." [1]
> 1. RFC 5375 - IPv6 Unicast Address Assignment Considerations, Section 
> 2.4

Like I said, /48 for residential networks is still standard practice.

I believe that the idea for a /56 first came from Geoff Huston in this

If you have a network architecture which requires you to assign /48s to
each POTENTIAL customer, then you are at risk of not qualifying for
additional blocks because ARIN only counts ACTUAL customers. But, if you
assign /56s to each POTENTIAL customer, you are almost certain to never
need to go back to ARIN.
Some cable providers need to assign a block to every residence that
their cable goes past, which is why the policy was changed to allow for
/56 assignments.

And, as the policy states, these are only guidelines.
Some ISPs will need to assign a /47 or /46 per customer because their
business model requires it. Maybe they specialise in connecting hotels
who have a complex subnet plan to accomodate guest wifi subnets, room TV
subnets, room telephony subnets, maid subnets per trolley, management
subnets per section of each floor, banquet room subnets, conference
subnets per kiosk, kitchen subnets, staff network subnets, bar wifi
subnet, restaurant staff subnets, and lord knows what else. ARIN cannot
dictate one size fits all, because increasingly, the ISP business is
diversifying and there are all kinds of strange things out there.

--Michael Dillon

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