It's Ars Tech's turn to bang the IPv4 exhaustion drum

Tue Aug 19 17:45:49 UTC 2008

On 8/19/08 1:36 PM, "Nathan Ward" <nanog at daork.net> wrote:

> 64 bits is not a magical boundary.
> 
> 112 bits is widely recommended for linknets, for example.
> 
> 64 bits is common, because of EUI-64 and friends. That's it.
> There is nothing, anywhere, that says that the first 64 bits is for
> routing.

Actually, there is text that says so:
RFC4291: IPv6 Addressing Architecture, section 2.5.1

"  For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long and to be
   constructed in Modified EUI-64 format."

The fact that most implementations ignore this is a different story.

In practice, many routers require the packet to go twice in the hardware if
the prefix length is > 64 bits, so even though it is a total waste of space,
it is not stupid to use /64 for point-to-point links and even for loopbacks!

  - Alain.