v6 subnet size for DSL & leased line customers

Iljitsch van Beijnum iljitsch at muada.com
Sat Dec 22 23:24:32 UTC 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.

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