v6 & DSL / Cable modems [was: Private use of non-RFC1918 IP space (IPv6-MW)]

Stephen Sprunk stephen at sprunk.org
Sat Feb 7 02:36:53 UTC 2009

Joe Abley wrote:
> On 4-Feb-2009, at 16:16, Patrick W. Gilmore wrote:
>> I guess I was thinking about v4 modems which do not get a subnet, 
>> just an IP address.  If we really are handing out a /64 to each DSL & 
>> Cable modem, then we may very well be recreating the same problem.
> All the advice I have heard about address assignment to broadband 
> subscribers is to give each subscriber a /56, in addition to the link 
> address (which is effectively a /128). The last time I looked, the v6 
> allocation of every RIR apart from ARIN recommended a /48 instead of a 
> /56.

I'm not sure that that counts as "advice".  Current ARIN policy takes no 
position as to which ISPs should do.

IIRC, the /56 allowance came at the behest of a particular cable ISP 
that wanted to assign addresses to every home passed, whether or not the 
residents signed up for service, but did not want to pay for the /24 or 
so that would be required if they were handing out /48s -- if ARIN would 
even accept that flimsy justification.  I can understand the technical 
benefits of pre-assignment, and I would have preferred that the policy 
(and fee schedule) were amended to better handle that case.

> I have been specifically advised against assigning a /64 per 
> subscriber on the grounds that it is short-sighted, since v6 
> residential gateways, when they come in large numbers, will expect to 
> be able to assign addresses to more than one subnet in the customer 
> network.

... which could be handled by giving out additional /64s via DHCP PD.  I 
would expect the majority of customers to need no more than two or 
perhaps three subnets, with a huge fraction of that needing only one 
(not including the /127 to the CPE device).

> I suspect that for many regional ISPs a single allocation sufficient 
> to number 16 million customers is probably good enough. In Canada, for 
> example, that's half the total population, and probably larger than 
> the total number of residences.
> No doubt there are a countable and significant number of super-ISPs in 
> larger markets (or spanning multiple markets) that have requirements 
> that out-strip that of a single /32, but I feel comfortable predicting 
> that they are the minority in the grand scheme of things (and in any 
> case, they can always request a larger allocation).

More importantly, we can see that in Europe, RIPE has been perfectly 
willing to hand out enormous allocations to such mega-ISPs.  A few in 
the US have also gotten larger-than-/32 allocations, and IMHO that is 
the "correct" route, not shrinking the size of customer assignments.  
There are more than enough prefixes to go around in the minuscule part 
of the IPv6 space we're currently using.  The real concerns should be 
(a) how we keep the number of prefixes per AS as close to 1.00 as 
possible, and  (b) how to deal with the explosion in the number of ASes 
due to multihomed leaf sites.  However, those problems are much harder, 
so some engineers are looking at how to solve problems that we already 
know how to solve "successfully" but don't actually matter.


Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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