It is because of IEEE EUI-64 standard. It was believed at the time of IPv6 development that EUI-48 would run out of numbers and IEEE had proposed going to EUI-64. While IEEE still hasn't quite made that change (though Firewire does appear to use EUI-64 already), it will likely occur prior to the EOL for IPv6. There is a simple algorithm used by IEEE for mapping EUI-48 onto the EUI-64 space. The 0x02 bit of the first octet of an EUI-64 address is an L-Flag, indicating that the address was locally generated (if it is a 1) vs. IEEE/vendor assigned (if it is a 0). The mapping process takes the EUI-48 address XX:YY:ZZ:RR:SS:TT and maps it as follows: let AA = XX xor 0x02. AAYY:ZZff:feRR:SSTT ff:fe above is literal. IPv6 was originally going to be a 32-bit address space, but, the developers and proponent of SLAAC convinced IETF to add 64 more bits to the IPv6 address for this purpose. Since bits are free when designing a new protocol, there really was no reason to impose such limitations. You really don't gain anything by going to /80 at this point. There are more than enough addresses available in IPv6 for any foreseeable future even with /64 subnets. Owen On Jun 6, 2012, at 7:58 AM, Chuck Church wrote:
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