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

Alexandru Petrescu alexandru.petrescu at gmail.com
Tue Jun 19 10:44:50 CDT 2012

I think, the length of Interface ID be 64 is so mostly because IEEE
works now with 64bit EUI identifiers (instead of older 48bit MAC
addresses).  I.e. compatibility between IEEE and IETF IPv6 would be the
main reason for this Interface ID to be 64.

And this is so, even though there are IEEE links for which the MAC
address is even shorter than 64bit, like 802.15.4 short addresses being
on 16bit.  For those, an IPv6 prefix length of 112bit would even make
sense.  But it's not done, because same IEEE which says the 15.4 MAC
address is 16bit says that its EUI is 64bit. (what 'default' fill that
with is what gets into an IPv6 address as well).

The good thing isthere is nothing in the RFC IPv6 Addressing
Architecture that makes the Interface ID to be MUST 64bit.  It just says

What there _is_, is that when using RFC stateless addess
autoconfiguration (not DHCP) and on Ethernet and its keen (WiFi,
Bluetooth, ZigBee, more; but not USB nor LTE for example) then one must
use Interface ID of 64bit; and consequently network prefix length of
64bit no more.


Le 06/06/2012 16:58, Chuck Church a écrit :
> 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

More information about the NANOG mailing list