On Wed, 06 Jun 2012 10:58:05 -0400, Chuck Church <chuckchu...@gmail.com>
wrote:
Does anyone know the reason /64 was proposed as the size for all L2
domains?
There is one, and only one, reason for the ::/64 split: SLAAC. IPv6 is a
classless addressing system. You can make your LAN ::/117 if you want to;
SLAAC will not work there.
The reason the requirement is (currently) 64 is to accomodate EUI-64
hardware addresses -- firewire, bluetooth, fibre channel, etc.
Originally, SLAAC was designed for ethernet and its 48bit hardware
address. (required LAN mask was ::/80.) The purpose wasn't to put the
whole internet into one LAN. It was to make address selection
"brainless", esp. for embeded systems with limited memory/cpu/etc... they
can form an address by simply appending their MAC to the prefix, and be
99.99999% sure it won't be in use. (i.e. no DAD required.) However, that
was optimizing a problem that never existed -- existing tiny systems of
the day were never destined to have an IPv6 stack, "modern" IPv6 hardware
can select an address and perform DAD efficiently in well under 1K. (which
is noise vs. the size of the rest of the IPv6 stack.)
SLAAC has been a flawed idea from the first letter... if for no other
reason than it makes people think "64bit network + 64bit host" -- and that
is absolutely wrong. (one cannot make such assumptions about networks they
do not control. it's even worse when people design hardware thinking that.)
--Ricky
