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

Joe Abley jabley at hopcount.ca
Thu Feb 5 07:34:57 UTC 2009

On 4-Feb-2009, at 22:59, Mikael Abrahamsson wrote:

> On Wed, 4 Feb 2009, Joe Abley wrote:
>> 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.
> What you do is that you do /40-44 per router or so and announce this  
> to the rest of the network, then internally from that you assign /48  
> to /56 per customer out of that block.
> IPv6 enables us to lower address use and get less routes in the core  
> network (both within the ISP and globally).

That has the down-side today that you require customers to support  
DHCPv6 PD in order to get their /56, though.

For some providers that might be simple to organise (e.g. business- 
focussed ISPs whose customers use a reliably-small set of CPE). For  
residential customers, however, I think it's far more likely to be  
problematic, and hence the idea of retaining the option for static  
configuration seems prudent, at least at this early stage in the game.

Static configuration means you need the assignment to be consistent  
regardless of which LNS the customer lands on.

I appreciate that this approach may well be a luxury only available to  
relatively small ISPs, and that the approach will quite possibly need  
to change in the future. But it seems like a reasonable way to go for  


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