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Re: v6 subnet size for DSL & leased line customers

  • From: Iljitsch van Beijnum
  • Date: Sat Dec 22 18:26:35 2007

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.