CIDR FAQ

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.