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

Steve Clark sclark at netwolves.com
Wed Jun 6 15:02:42 CDT 2012

On 06/06/2012 03:05 PM, Owen DeLong wrote:
> 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.
> ff:fe above is literal.
> IPv6 was originally going to be a 32-bit address space, but, the developers
did you mean "originally going to be a 64-bit address space"...
> 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

Stephen Clark
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at netwolves.com

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