Owen DeLong owen at
Tue Dec 22 17:47:44 UTC 2015

> On Dec 22, 2015, at 01:21 , Bjørn Mork <bjorn at> wrote:
> Owen DeLong <owen at> writes:
>>> On Dec 20, 2015, at 08:57 , Mike Hammett <nanog at> wrote:
>>> The idea that there's a possible need for more than 4 bits worth of
>>> subnets in a home is simply ludicrous and we have people advocating
>>> 16 bits worth of subnets. How does that compare to the entire IPv4
>>> Internet?
>> I have more than 16 subnets in my house, so I can cite at least one
>> house with need for more than 4 bits just in a hand-coded network.
>> Considering the future possibilities for automated topological
>> hierarchies using DHCP-PD with dynamic joining and pruning routers, I
>> think 8 bits is simply not enough to allow for the kind of flexibility
>> we’d like to give to developers, so 16 bits seems like a reasonable
>> compromise.
> Thanks for summarizing why /48 for everybody is possible.  But I fear
> that is not helping much against arguments based on "need". I believe it
> is difficult to argue that anyone needs any IP address at all, given
> that there are lots of people in the world who seem to survive just fine
> without one…

Arguments based on “need” don’t make any sense in an IPv6 context.

Sure, we shouldn’t be so profligate in our distribution of the address pool
that we run out well before the protocol’s useful life is exhausted, but I
think I’ve shown that the current allocation policies, including /48 have
adequate protection against that occurring.

Being more restrictive just for the sake of being more restrictive doesn’t
serve any purpose. It doesn’t help anyone. As such, I just don’t understand
those arguments. If someone can show me a tangible benefit from a more
restrictive policy, I’m open to considering it, but so far, none exists.

> So, with that sorted out, let's consider what you can do with 16 bits of
> subnets.  One example is checksum neutral prefix translation (RFC6296)
> without touching the interface id bits . Let's say you have two upstream
> ISPs handing you the prefixes A/48 and B/56.  Neither offer any
> multihoming support to residential users and both do BCP38 of course. So
> you use B/56 internally and do prefix translation to allow your router
> to select upstream without involving the clients.  Thanks to the A/48
> from the first ISP, you are able to choose a set of 256 (or possibly 255
> since 0xffff cannot be used) checksum neutral subnet pairs.

That’s a really icky alternative to simple BGP multihoming (which is what
I’m currently using at home).

Of course, not the worst, but a significantly bad part of this is the provider
that’s only giving you a /56 to begin with. ;-)

> Yes, I know. Evil. No need. No CPE support.  Etc.

True that.

> The important part is that 16 bits of subnets is enough to play
> algorithmic tricks with the subnet part of your address too, whereas
> this is much more difficult with fewer bits.  No, you don't need to do
> it.  But you CAN.  The sparse IPv6 addressing model is about opening up
> possibilities.  Note that those possibilities includes restricting
> yourself to using a single address.  You don't have to use all your 2^80
> addresses :)

I completely agree.

> And for the ISPs, using /48 for every user means fewer prefix lengths to
> consider for routing and address management. Sure, we manage diverse
> prefix lengths in IPv4 today, but why not take advantage of this
> possible simplification if we can? Only those living on bugs will object
> to simpler address databases and routing filters.

Again, you’re preaching to the choir.


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