Host developers and users do care what the prefix length is.
There are lots of things you can do if you know that the network will never
run out of addresses. For example, you can form new addresses at will and
use them for anything you want (privacy; per-application addresses, or
whatever else you can think of). Read RFC 7934 for a list of the things
we've thought of so far, and bear in mind that if this IPv6 thing even only
lasts as long as IPv4, that's still several decades, and I'm sure we'll
have lots of bright ideas during that time if we don't shortsightedly carry
over IPv4 practices motivated by address scarcity.
A fixed prefix length is also very beneficial because it allows hosts to
extend the network indefinitely at layer 2 without giving up the benefits
provided by autoconfiguration and end to end connectivity. It means that
ill-informed or ill-intentioned network administrators cannot use
addressing to constrain apps in a way that leads to suboptimal user
experience. That sort of thing obviously does not happen in the 2914 or
15169 backbone. It does happen in lots of other places.
On Wed, Feb 22, 2017 at 1:44 PM, Christopher Morrow <
christopher(_dot_)morrow(_at_)gmail(_dot_)com> wrote:
On Tue, Feb 21, 2017 at 10:51 PM, Lorenzo Colitti
<lorenzo(_at_)google(_dot_)com>
wrote:
On Wed, Feb 22, 2017 at 12:12 PM, Christopher Morrow <
christopher(_dot_)morrow(_at_)gmail(_dot_)com> wrote:
But the configuration cost and management overhead is not proportional
to the hosts that are served by those interconnections, it is proportional
to the number of interconnections. A 10x100G peering interconnection that
serves X million hosts is one interface that has to be managed.
isn't the dicsussion here really:
"If you want to use /64 go ahead, if you want to use /121 go for it,
if you want to use SLAAC you'll get a /64 and like it"
Not sure. I for one wouldn't agree with that position, because I don't
see that /121 has enough advantages over /127 and /64 - and few enough
downsides for general-purpose hosts - to make it a good idea in general.
I don't think /121 is anymore special than /127... or /64. My point was we
don't care what prefix people use, generally, that there are cases where a
/64 is required and that's fine, there are cases where /64 isn't and people
can do what they want there. It's simple enough to do SLAAC/64 on lans and
other places.
Requiring /64 or /127 and nothing else means when you do have to do a /120
or something else you MAY end up fighting vendor problems because they made
assumptions about: "only ever 64 or 127".