Re: ISP customer assignments

[Bill Stewart <nonobvious () gmail com>]

If you've got an addressing system with enough bits that you don't
have to start stealing them, it makes sense to pick some boundary
length between
             our-problem  :  their-problem
128 bits is long enough, and changing protocols is nasty enough, that
it should let you Never Have To Do It Again.

Originally with IPv6, the boundary was a nice round 64 bits, with the
ISP on the network side and MAC48-based autoaddressing on the user
side, with the user side looking suspiciously similar to Novell
Netware and giving ~64K subnets, and this was back before DHCP had
taken over the world and it was expected that all kinds of weird
little toasters would be using IPv6.
But some relatively sensible people proposed using the (ugly) EUI-64
64-bit MAC, because they Never Wanted To Have To Do This Again Either.
 And unfortunately, because it's a good enough idea to be worth
accepting, it pushes the network boundary somewhat to the left,
because it's pretty obvious that an average household may have
multiple devices that want to autoconfigure themselves, so you
probably will end up needing multiple subnets.  And unfortunately,
there's no obviously correct boundary, and no particular reason for
all ISPs to use the same boundary, so there are endless arguments
about it on NANOG and elsewhere.

In general, /48's big enough for most large complex businesses (except
ISPs), and /60's more than big enough for a household and for many
small businesses, but we've got enough bits that it's worth using
octet-aligned addresses, so /56 is the magic number for them, except
at ISPs that simply don't want to bother giving out anything except
/48s.  There may be special cases for assigning /64s to end users,
such as IPv6-equipped cell phones, but that's a matter for specialized
carriers to provide, or for internal network managers at enterprises.
And if you're big enough to get Provider Independent Address Space and
an AS#, you're big enough to have a /48 of your own.

Now, IPv6 was supposed to allow the development of other
indistinguishable-from-magically advanced technology, such as getting
rid of the growth of routing tables by convincing everybody to be
happy with hierarchically assigned provider-aligned address space, and
unfortunately that hasn't matched the needs of businesses, which need
multihoming for reliability (so they'll be non-provider-aligned for at
least n-1 of their ISPs), plus want the ability to take their address
space with them when they change ISPs (because there are too many
devices and applications that insist on having hard-coded IP addresses
instead of using DNS, and because DNS tends to get cached more often
than you'd sometimes like.  So that problem shows no sign of going
away (in spite of shim6..)