It's Ars Tech's turn to bang the IPv4 exhaustion drum

Tue Aug 19 18:28:00 UTC 2008

>-----Original Message-----
>>> On Tue, 19 Aug 2008, michael.dillon at bt.com wrote:
>>>>> I don't have a problem with assigning customers a /64 of v6 space.
>>>>
>>>> Why so little? Normally customers get a /48 except for residential
>>>> customers who can be given a /56 if you want to keep track of
>>>> different block sizes. If ARIN will give you a /48 for every
>>>> customer, then why be miserly with addresses?
>>> I don't operate an ISP network (not anymore, anyway...).  My
>>> customers are departments within my organization, so a /64 per
>>> department/VLAN is more sane/reasonable for my environment.
>>
>> Uh, the lower 64 bits of an IP6 address aren't used for routing you
>> know? They're essentially the mac address, or some other sort of
>> autoconf'd host identifier. Last I heard, the smallest allocation is
>> supposed to be a /48 -- I hadn't heard of the /56 thing that Michael
>> was speaking of, though I'm not surprised. There's 64 bits for
>> routing... no need to be so stingy :)
>
>
>64 bits is not a magical boundary.
>
>112 bits is widely recommended for linknets, for example.
>
>64 bits is common, because of EUI-64 and friends. That's it.
>There is nothing, anywhere, that says that the first 64 bits is for
routing.

Just to be clear - this http://tools.ietf.org/html/rfc4291#section-2.5.4
does say:
"   All Global Unicast addresses other than those that start with binary
   000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as
   described in Section 2.5.1.  Global Unicast addresses that start with
   binary 000 have no such constraint on the size or structure of the
   interface ID field."

(And again - this is a case where the real world and the IETF may not agree
100% ...)

/TJ