[ppml] Policy Proposal: IPv6 Assignment Guidelines

michael.dillon at bt.com michael.dillon at bt.com
Thu Aug 23 10:49:21 EDT 2007

> Also, why the extra bit, we've decided with HD Ratio that as 
> you get larger you should have some extra bits here and there 
> to allow you to allocate efficiently internally, so it's more 
> likely you'll fit in a single block.  If you're worried about 
> that, this is not the place to worry.  Going from a /45 to a 
> /44, inside of a PA block that's nicely aggregated is not 
> such a big deal.  Going from a /32 to a /19 due to the 
> effects of HD ratio on larger providers is probably a better 
> place to start.

These skinflint measures just go against the grain. IPv6 was designed so
that everyone, except the largest ISPs, gets more addresses than they
could ever possibly need. That is so that everyone, except the largest
ISPs, never needs to go back to their address provider for more
addresses. That means that everyone, has spare subnets to carve off a
few for some special project such as an in-law apartment.

Additionally, IPv6 was designed so that everyone, except a few of the
largest (and unusual) cases, gets the same size of allocation. When ARIN
and APNIC went to /56 for consumer subscribers (homes and apartments)
that didn't change this principle, just added another class of
installation that is fairly clearly defined in the real world.

The unusual exceptions are very large ISPs who might need a /20, not a
/32. And a few campus-type end-user sites with lots of IT infrastructure
that might need *TWO* /48s instead of one. I leave off data centers
because I expect them to get a /32 and hand out a /48 to every customer,
or maybe every rack.

It is *NOT* wasteful to give an IPv6 installation more addresses than
they will ever use. That is, in fact, by design. It allows any point in
the network to maintain a surplus of unused subnets so that this point
in the network can be expanded *WITHOUT RENUMBERING OR RESTRUCTURING* of
the surrounding network.

HD ratios have one useful purpose, and only one. That is to place a
limit on the very largest ISPs who ask for more than one /32 allocation.
It is there to prevent people from going wild, otherwise 20 of the
world's largest providers could all make plans to server 25% of the
world's population and all request far more /32s than they could ever

In fact, there is no guarantee that the HD ratio rules will *EVER* be
applied. Once IPv6 really kicks off (and it is starting to appear in
RFPs with increasing frequency), people will get real-world experience
with deployment, and I fully expect ISPs to change the RIR policies
before anybody exhausts their first IPv6 allocation.

--Michael Dillon

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