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In message <AANLkTinrhPYXvtS5wtA0PuhtEmi3f4tN9J5KOCBF1a=5 at mail.gmail.com>, Mart
in Millnert writes:
> Jeremy,
> I have not heard of any IP stack that is built to accept 240/4.
> Neither Linux 2.6.37 nor Windows 7 accepts it, and let's not think
> about all routers, including CPE:s, out there.
> The logic goes:
> You are many orders of magnitudes more likely to get v6 off the
> ground, than 240/4 or 224/4 as unicast IPv4.  224/3 will never be very
> usable as public v4 space since every non-upgraded host on the
> Internet will be unable to send packets to them, eg, for every
> additional host you introduce with these addresses the worse the
> reachability situation becomes for the v4 Internet. Notably, this is
> the inverse of what happens when you introduce more hosts with native,
> proper IPv6, in the IPv6-Internet.
> Cheers,
> Martin

The lines of code to make 240/4 work as unicast << loc to add IPv6
and will usually fit into the amount of flash already on the CPE
box.  It's a surgical change rather than add a whole new stack. It
might even be possible to convice the CPE vendors to make new images
for old hardware.

You also don't need to make it work with the whole world.  Just
between the CPE and the LSN and/or 6rd border router.  15 /8's
(leave 255 alone) is a lot of space for a ISP to use.  The CPE would
signal support (e.g. a DHCP option) and the ISP would only return
class E addresses if it was sure the path was clean.  Those that
need a public address would clear the option.  It would default on.

With luck the asic will support unicast in 240/4 space so you get
fast path for IPv6 using 6rd on IPv4 only routers.

This model also allows it to be deployed incrementally.  This is a
software upgrade rather than a hardware upgrade.

If you constrain the problem space it becomes more managable problem.

The question is do you want to have to deploy the same address space
multiple times or is it worth a little bit of co-ordinated effort
to do this.

IPv4 + IPv6 [NAT/6RD] E space + public IP4 [NAT/6RD] RFC 1918 space + IPv6

It can also be used to deliver IPv6 only over 6rd.

IPv4 + IPv6 [AFTR/6RD] E space [B4/6RD] RFC 1918 + IPv6  (double encap)
IPv4 + IPv6 [NAT64/6RD] E space [6RD] IPv6               (single encap)

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org