IPv6 end user addressing

William Herrin bill at herrin.us
Mon Aug 8 05:15:29 UTC 2011

On Sun, Aug 7, 2011 at 6:09 PM, Jonathon Exley
<Jonathon.Exley at kordia.co.nz> wrote:
> This has probably been said before, but it makes me uncomfortable to think of everybody in the world being given /48 subnets by default.
> All of a sudden that wide expanse of 2^128 IP addresses shrinks to 2^48 sites. Sure that's still 65535 times more than 2^32 IPv4 addresses, but wouldn't it be wise to apply some conservatism now to allow the IPv6 address space to last for many more years?
> After all, there are only 4 bits of IP version field so the basic packet format won't last forever.

Hi Jonathan,

IPv6 uses a slightly different mental model when it comes to address allocation.

In IPv4 you assigned blocks of 32-bit addresses. In IPv6 this is no
longer the case. You do not assign blocks of 128-bit addresses.

In IPv6 you assign blocks of 64-bit LANs. Each LAN is 64-bits long as
required by technologies like SLAAC. So, a /48 is 65k LANs, _not_
however many bazillion addresses.

The question you should be asking is: is it excessive or unwise to go
around assigning 65,000 LANs to every customer? Would 256 (/56) or 1
(/64) be a better number?

Assigning 1 LAN seems questionable. We're trying to avoid NAT with
IPv6 which means a customer may want to spend a LAN on things like the
interior network of a virtual machine server. It's hard to do this if
you only have one LAN.

On the other hand, a whole lot of folks have through this through
before you and concluded that a /56 (256 LANs per customer) is a
smarter starting point than a /48.

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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