CIDR FAQ

   cc: rv at zeus.nic.dtag.de, owen at delong.sj.ca.us, inet-access at earth.com,
	   nanog at merit.edu, local-ir at ripe.net
   Date: Thu, 17 Aug 95 06:04:21 PDT
   From: Yakov Rekhter <yakov at cisco.com>

   David,

   > I thought that the IPv6 address space had fields for a provider number
   > outside of the "legacy" IPv4 address space.  Thus "legacy" IPv4
   > networks can be moved by changing the provider bits in the larger IPv6
   > address.
   > 
   > The routers involved will need to be IPv6 capable, but the individual
   > hosts on a lan continue to use their old IPv4 addresses and do *not*
   > need to be renumbered.  The IPv4 networks are now "mobile" and can
   > move from provider to provider providing the routers involved all talk
   > IPv6.

   In IPv6 there is a notion of "IPv4 compatible" addresses. These are
   IPv6 addresses (128 bits) that have IPv4 address as their low order 32
   bits and the rest (96 bits) are zero. If you look at the IPv6
   transition then you'll find that it is a so-called "dual stack"
   transition, where hosts have to have both IPv4 and IPv6. Moreover, to
   make transition reasoably simple, hosts should have IPv6 addresses that
   are "IPv4 compatible".

   A consequence of this, is that the allocation of IPv6 addresses that
   are "IPv4 compatible" is *exactly the same* as the current IPv4
   allocation. So, "legacy networks" would not be able to move "by
   changing the provider bits in the larger IPv6 address", as "IPv4
   compatible" addresses have no "provider bits".

   What you said is correct only if hosts use IPv6 addresses that
   are not "IPv4 compatible". But transition with hosts that don't have
   IPv4 compatible addresses is quite messy.

   Yakov.

But isn't it possible to have a router convert from an IPv6 address
with provider bits to an IPv4 address by simply stripping all but the
lowest 32 bits from the IPv6 address?

The hosts on the LAN remain entirely IPv4, but the addresses that
leave the router have the provider bits added by the router.

Isn't that one of the goals of IPv6?  To allow hosts to remain IPv4
and coexist with the IPv6 hosts and backbone?

-- 
David.Schmidt at on-ramp.ior.com  Internet On-Ramp, Inc.    (509)624-RAMP (7267)
Spokane, Washington            http://www.ior.com/	 (509)622-2866 (fax)