[ppml] IPv6 addressing, allocation, and subnets
briand at ca.afilias.info
Sun Nov 18 19:58:17 EST 2007
michael.dillon at bt.com wrote:
>> It is time to get with the CIDR program, not to advocate the
>> antiquated class-ful paradigm.
> CIDR is IPv4. We are discussing IPv6 which is different.
No, I'm afraid you're wrong on this point.
CIDR is classless inter-domain routing. It is neither IPv4 nor IPv6.
The basic concept underpinning CIDR is, longest match wins when
comparing two prefixes.
Without it, CIDR would not have worked, or not have scaled.
With it, covering aggregates and variable-length subnet masks (VLSM) are
possible, and provide the basic tools for scaling both internal and
external routing domains.
And, as it happens, IPv6 *is* CIDR-oriented.
The primary RFC for IPv6 specifically excludes any requirement for
specific structure within IPv6 addresses, when viewed by routers.
(Currently this is RFC 4291.)
It does anticipate that routers will have knowledge of hierarchical
routing schemes (examples would include OSPF or ISIS, with stub or
And, while IPv6 supports the notion of fixed-size interface identifier
(II) (currently fixed at a size of 64 bits for global unicast prefixes),
the II is used for link-local addressing and node address
Static host address configuration, and DHCPv6, support prefixes of
arbitrary length, and are not restricted to /64 prefixes.
So, your contention that IPv6 is "different", and not CIDR, is patently
(Link-local addressing is a clever alternative to ARP, and one of the
better elements of IPv6. However, IPv6 does not require that all IPv6
addresses on an interface use the EUI-64 value as the host part of the
address; it does specify that when doing autoconfiguration, the
interface identifier is used for the host portion.)
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