v6 subnet size for DSL & leased line customers

Randy Bush randy at psg.com
Thu Dec 27 04:19:27 UTC 2007

>> 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
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.

personally, i agree that one subnet is likely to be insufficient in a
large proportion of cases.  so keeping to the /64 per subnet religion, a
/64 per site is insufficient for the default.

still personally, i think the one /64 subnet per device is analogous to
one receptacle per mains breaker, i.e. not sensible.

> there are three legs to the tripod
> 	network operator
> 	user
> 	equipment manufacturer
> They have (or should have) a mutual interest in:
> 	Transparent and automatic configuration of devices.

as you have seen from chris's excellent post [0] on this one, one size
does not fit all.  this is likely another worthwhile, but separate,

> The assignment of globally routable addresses to internet
> connected devices

i suspect that there are folk out there who equate nat with security.  i
suspect we both think them misguided.

> The user having some control over what crosses the boundry
> between their network and the operators.




[0] - <http://www.merit.edu/mail.archives/nanog/msg04887.html>

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