v6 subnet size for DSL & leased line customers

David Barak thegameiam at yahoo.com
Sun Dec 23 20:44:10 UTC 2007

-- On Sun, 12/23/07, Chris Adams <cmadams at hiwaay.net> wrote:

> From: Chris Adams <cmadams at hiwaay.net>
> Subject: Re: v6 subnet size for DSL & leased line customers
> To: nanog at merit.edu
> Date: Sunday, December 23, 2007, 2:21 PM
> Once upon a time, Florian Weimer <fw at deneb.enyo.de>
> said:
> > >> Right now, we might say "wow, 256
> subnets for a single end-user... 
> > >> hogwash!" and in years to come,
> "wow, only 256 subnets... what were we 
> > >> thinking!?"
> > >
> > > Well, what's the likelihood of the "only
> 256 subnets" problem?
> > 
> > There's a tendency to move away from (simulated)
> shared media networks.
> > "One host per subnet" might become the norm.
> So each host will end up with a /64?
> How exactly are end-users expected to manage this?  Having
> a subnet for
> the kitchen appliances and a subnet for the home theater,
> both of which
> can talk to the subnet for the home computer(s), but not to
> each other,
> will be far beyond the abilities of the average home user.

As I see it, one of the big benefits IPv4 provided was logical addresssing in an easy-to-understand and easy-to-aggregate manner, with small layer-2 networks divided by routers.  What we've gone to with IPv6 is a gigantic layer-2 network (the flat autoconfiguration space).  

I think we got here when "site-local" went away - we've effectively redefined link-local to mean "site-local," while using globally unique addressing.

Personally, I don't relish the idea of millions of hosts participating in spanning-tree, so I'd rather see us move back toward the direction of using layer-3 addresses to break up layer-2 islands.

How about this for a modest proposal for a capability:
Allow autoconfigured generation of IPv6 interface addresses to use this format:

(one byte VLAN ID) (48 bit MAC address)

instead of:

(24 bit half-mac) (FFFE) (24 bit half-MAC)

This would allow a CPE router to serve as the gateway for up to 64K VLANs, and wouldn't waste a byte in the middle of the address space.

How about it?

David Barak
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