v6 subnet size for DSL & leased line customers

Fri Dec 28 19:22:06 UTC 2007

Randy Bush wrote:
>>> vendors, like everyone else, will do what is in their best interests.
>>> as i am an operator, not a vendor, that is often not what is in my best
>>> interest, marketing literature aside.  i believe it benefits the ops
>>> community to be honest when the two do not seem to coincide.
>> If the ops community doesn't provide enough addresses and a way to use
>> them then the vendors will do the same thing they did in v4.
> 
> i presume you mean nat v6/v6.  this would be a real mess and i don't
> think anyone is contending it is desirable.  but this discussion is
> ostensibly operators trying to understand what is actually appropriate
> and useful for a class of customers, i believe those of the consumer,
> soho, and similar scale.
> 
> to summarize the positions i think i have heard
>   o one /64 subnet per device, but the proponent gave no estimate of the
>     number of devices
>   o /48
>   o /56
>   o /64

It plausible that if one were to assign a single /64 and reserve a 56 to
delegate per customer that you could number about 16 million customers
per /32 with a few hundred thousand /64s remaining for infrastrucuture.
size of an agregate for a pop might be /48 (~250 customers) to /40 (65k
customers) to /36 (1 million customers)

A large retail isp might under those circumstances be able to get away
with order of /28 to /30 in total.

> the latter three all assuming that the allocation would be different if
> the site had actual need and justification.
> 
> personally, i do not see an end site needing more than 256 subnets *by
> default*, though i can certainly believe a small minority of them need
> more and would use the escape clause.  so, if we, for the moment, stick
> to the one /64 per subnet religion, than a /56 seems sufficient for the
> default allocation.
> 
> personally, i have a hard time thinking that any but a teensie minority,
> who can use the escape clause, need more than 256.  hence, i just don't
> buy the /48 position.