IPv6 Default Allocation - What size allocation for Loopback Address

Mon Oct 13 00:04:34 UTC 2014

In message <CAP-guGXezFUSSpDznCtb6DZNXpV=2RBdWGH+sf-xSdKj_mCsiw at mail.gmail.com>, William Herrin writes:
> On Sat, Oct 11, 2014 at 1:41 AM, Faisal Imtiaz <faisal at snappytelecom.net> wrote:
> > A follow up question on this topic..
> >
> > For Router Loopback Address .... what is wisdom in allocating a /64 vs /128 ?
> > (the BCOP document suggests this, but does not offer any explanation or merits of one over the other).
> 
> Hi Faisal,
> 
> One of the viewpoints held by some in the IETF is that an IPv6 address
> is not 128 bits. Rather, it is 64 bits of network space and 64 bits of
> host space. I'm told this viewpoint is responsible for the existence
> of a 128 bit address instead of IPv6 using 64 bit addresses.

IPNG looked at 48 bits, 64 bits and 128 bits addresses.  48 and 64
bits would both have left everyone tightly managing subnet sizes
and allocation sizes like we do in IPv4.  IPv6 went to 128 bits to
*allow* for a 64/64 split eventually where one didn't have to tightly
manage subnet sizes and allocations.  Earlier plans looked at 48
bits for the subnet size based in Ethernet MAC.  It went to 64 bits
with 48 bit MAC's padded to 64 bits to account for 64 bit MAC's and
because a 64/64 split would possibly be more efficient / simpler.

No one was making it a hard split at the time.

> If you follow that reasoning, the subnet mask should always be /64, no
> matter where the address is assigned.
> 
> There are, of course, excellent operational reasons not to religiously
> follow that plan.
> 
> Regards,
> Bill Herrin
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Mark Andrews, ISC
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