Re: IPv6 /64 links (was Re: ipv6 book recommendations?)

["Ricky Beam" <jfbeam () gmail com>]

On Wed, 06 Jun 2012 10:58:05 -0400, Chuck Church <chuckchurch () 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