[arin-discuss] IPv6 End User Assignments

[michael.dillon at bt.com]

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

None.
"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:
<http://smakd.potaroo.net/ietf/idref/draft-narten-ipv6-3177bis-48boundar
y/index.html>

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 analysis:
<http://www.potaroo.net/ispcol/2005-07/ipv6size.html>

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