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

Wed Jun 6 19:05:31 UTC 2012

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