[101205] in North American Network Operators' Group
Re: v6 subnet size for DSL & leased line customers
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Sat Dec 22 18:25:51 2007
Cc: Deepak Jain <deepak@ai.net>, Joe Greco <jgreco@ns.sol.net>,
nanog@merit.edu
From: Iljitsch van Beijnum <iljitsch@muada.com>
To: Ross Vandegrift <ross@kallisti.us>
In-Reply-To: <20071222202317.GB3844@kallisti.us>
Date: Sun, 23 Dec 2007 00:24:32 +0100
Errors-To: owner-nanog@merit.edu
On 22 dec 2007, at 21:23, Ross Vandegrift wrote:
> IPv6 documents seem to assume
> that because auto-discovery on a LAN uses a /64, you always have to
> use a /64 global-scope subnet. I don't see any technical issues that
> require this though. ICMPv6 is capable of passing info on prefixes of
> any length - prefix length is a plain old 8bit field.
> In fact, until I read the ARIN documents to receive an assignment at
> work, I assumed this would be how people would operate. So what's the
> concern? Give all end users a /64 and let them subnet that as they
> see fit. If DHCPv6 would take care of it automatically with shorter
> prefixes, that's fine
First of all, there's RFC 3513:
For all unicast addresses, except those that start with binary value
000, Interface IDs are required to be 64 bits long and to be
constructed in Modified EUI-64 format.
Second, we currently have two mechanisms to configure IPv6 hosts with
an address: router advertisements and DHCPv6. The former has been
implemented in ALL IPv6 stacks but doesn't work if your subnet isn't
a /64. The latter is (I think) available on the client side in Windows
Vista. There are a few DHCPv6 server implementations, but the ones I
tested 2 years ago wouldn't do address assignment. (You still need the
router advertisements to learn your default gateway and prefix length
as DHCPv6 can't tell you those.) So although many people want to stick
to the DHCP model they know from IPv4, that's extremely hard to do
with IPv6 the way things currently are.
