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.tremblay...@videotron.com > [mailto:jean-francois.tremblay...@videotron.com] > Sent: Wednesday, June 06, 2012 10:36 AM > To: an...@huge.geek.nz > Cc: NANOG list > Subject: IPv6 /64 links (was Re: ipv6 book recommendations?) > > Anton Smith <an...@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 > > >