v6 subnet size for DSL & leased line customers

Mohacsi Janos mohacsi at niif.hu
Sun Dec 23 08:49:04 UTC 2007

On Sun, 23 Dec 2007, Iljitsch van Beijnum wrote:

> 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.

Actually we tested DHCv6 implementation 2-3 years ago.

The dibbler seemed to be rather complete DHCPv6 implementation. I think 
default gateway and prefix length distribution via DHCPv6 will be quite 
problematical any many situation. There plenty of organisation who has a 
dedicated team/person for network management (routers, switches etc.), 
while another team/person for system management (dhcp, servers etc.). So 
configuring DHCPv6 requires cooperation which takes time, but users are 

Best Regards,
 		Janos Mohacsi

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