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

["Chuck Church" <chuckchurch () gmail com>]

Does anyone know the reason /64 was proposed as the size for all L2 domains?
I've looked for this answer before, never found a good one.  I thought I
read there are some L2 technologies that use a 64 bit hardware address,
might have been Bluetooth.  Guaranteeing that ALL possible hosts could live
together in the same L2 domain seems like overkill, even for this group.
/80 would make more sense, it does match up with Ethernet MACs.  Not as easy
to compute, for humans nor processors that like things in 32 or 64 bit
chunks however.  Anyone have a definite answer?

Thanks,

Chuck

-----Original Message-----
From: Jean-Francois.TremblayING () videotron com
[mailto:Jean-Francois.TremblayING () videotron com] 
Sent: Wednesday, June 06, 2012 10:36 AM
To: anton () huge geek nz
Cc: NANOG list
Subject: IPv6 /64 links (was Re: ipv6 book recommendations?)

Anton Smith <anton () huge geek nz> a écrit sur 06/06/2012 09:53:02 AM :

> Potentially silly question but, as Bill points out a LAN always 
> occupies a /64.
>
> Does this imply that we would have large L2 segments with a large 
> number of hosts on them? What about the age old discussion about 
> keeping broadcast segments small?

The /64 only removes the limitation on the number of *addresses* on the L2
domain. Limitations still apply for the amount of ARP and ND noise. A
maximum number of hosts is reached when that noise floor represents a
significant portion of the link bandwidth. If ARP/ND proxying is used, the
limiting factor may instead be the CPU on the gateway. 

The ND noise generated is arguably higher than ARP because of DAD, but I
don't remember seeing actual numbers on this (anybody?). 
I've seen links with up to 15k devices where ARP represented a significant
part of the link usage, but most weren't (yet) IPv6. 

/JF