v6 subnet size for DSL & leased line customers

Ross Vandegrift ross at kallisti.us
Sun Dec 23 17:54:34 UTC 2007

On Sun, Dec 23, 2007 at 12:24:32AM +0100, Iljitsch van Beijnum wrote:
> 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.

Ahhh, thanks - that is the only thing I have ever seen that gives any
reason for the /64 prefix.  Sadly, the document contains no
compelling technical reasons for it - looks like it's done just so
things are easy when generating interface IDs from ethernet addresses.

> 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.

But the protocols don't imply or require this.  All of the messages
used in stateless autoconfig will behave as expected with longer prefix
lengths.  So it seems that because the interface identifier has to be
64-bits, stateless autoconfig is unnecessarily crippled.

For kicks I just tried RAs with a /96 prefix.  Linux 2.6 checks and
enforces the requirement from RFC3513, though it'd be trivial to
change.  But I'm guessing other vendors enforce this as well.

Ross Vandegrift
ross at kallisti.us

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
	--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37

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