IPv6 Addressing Help

William Herrin herrin-nanog at dirtside.com
Fri Aug 14 13:33:10 CDT 2009

On Fri, Aug 14, 2009 at 11:03 AM, Chris Gotstein<chris at uplogon.com> wrote:
> We are a small ISP that is in the process of setting up IPv6 on our network.
>  We already have the ARIN allocation and i have a couple routers and servers
> running dual stack.  Wondering if someone out there would be willing to give
> me a few pointers on setting up my addressing scheme?  I've been mulling
> over how to do it, and i think i'm making it more complicated than it needs
> to be.  You can hit me offlist if you wish to help.  Thanks.

Hi Chris,

Suggested scheme:

Router loopback: /128
Router serial link: /126
Router/server ethernet link: /64
Dynamic IP customer: /128 from a /64 pool

Dynamic IP always-on customer: Not sure there are any well conceived
and solidly implemented answers here.

Your customer's "DSL router" isn't going to work and you shouldn't
expect a production-grade IPv6 NAT CPE any time soon. You can go DHCP
or autoconfiguration and let him chew as many /128's as he wants but
then you'll run into the broadcast traffic problem same as when you
used DHCP for IPv4. On the flip side, you can convert your always-on
folks to static IP customers with the risk of a routing explosion as
these customers move around and as you merge and split service POPs.

I'm not aware of any way of dynamically assigning an IPv6 subnet to a
customer that's as well automated as IPv4 /32 dynamic assignment to a
DSL router with an RFC1918 NATed interior, but that may just be my
ignorance since I haven't needed to research it.

Static IP customer: /60
Any static-IP customer who bothers to ask: /48

In all other respects follow whatever strategy works for you for IPv4
wrt routing areas and aggregation.

Several notes:

The RDNS delegation boundary for IPv6 is 4 bits (as opposed to IPv4's
8 bits). This makes boundaries like /48, /52, /56, /60 and /64 very
convenient. You should probably avoid customer assignments that don't
fall on one of those boundaries.

Ethernet in IPv6 is intended to work on a /64 subnet. You can make it
work on any other size but why create extra hassle for yourself for no
good reason?

I recommend /60 as the customer default where most folks suggest /56
or /48. The IPv6 use profile looks a heck of a lot like the IPv4 use
profile and /60 is 16 subnets. How many of your customers find a
reason to use more than 3 IPv4 subnets, including their RFC1918 ones?
Relatively few.

Giving every customer enough subnets by default to meet 90% of the
typical usage profiles is not the worst idea in the world... IMHO it's
a pretty bright idea. But there's no need to be damnfool wasteful
about it.

Bill Herrin

William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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