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

Joe Abley jabley at hopcount.ca
Thu Feb 5 06:37:00 UTC 2009

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 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  

> And before anyone says "there are 281474976710656 /48s!", just  
> remember your history.

The pertinent numbers given the thinking above are 2^24 == 16,777,216  
customer /56 assignments , or 2^16 == 65536 customer /48 assignments  
per /32 allocation from an RIR.

(If a /32 is all you have, then you will want to reserve some of that  
space for your own infrastructure, and the numbers above will be  
slightly higher than reality.)

I see people predicting that giving everybody a /56 is insane and will  
blow out routing tables. I don't quite understand that; at the  
regional ISP with which I am most familiar 40,000 or so internal/ 
customer routes in BGP, and I have not noticed anything fall over.  
This is 2008: we are not dealing with routers maxed out with 256MB of  
RAM. And this is without any attempt to aggregate per LNS, or per POP.

(This regional ISP is close to being able to provide responses to  
IPV6CP requests for all customers to establish an interface id, ND/RA  
to assign a link address, and DHCPv6 PD on request, for all customers;  
it's working in the lab, but hasn't yet been rolled out on the  
production access routers, which are all Juniper E-series devices. No,  
there's no direct revenue in it today; yes, the vast majority of  
customers are probably using XP or a residential gateway that will  
never talk v6.)

If you need to worry about the impact on your internal routing tables  
of internal customer growth, then it seems you should be more worried  
about the impact on your routing tables of growth in the global v4  
table (which is surely more rapid, and arguably can be expected to  
accelerate as v4 exhaustion leads to imaginative inter-organisation  
address assignment for fee).

> I was not there when v4 was spec'ed out, but I bet when someone said  
> "four-point-two BILLION addresses", someone else said "no [email protected]#%'ing  
> way we will EVER use THAT many...."

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).


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