[arin-ppml] A modest proposal for IPv6 address allocations

michael.dillon at bt.com michael.dillon at bt.com
Tue Jun 2 05:31:54 EDT 2009

> Ah I see. The typical residences will need 
> 4,722,366,482,869,640,000,000 addresses, whereas your typical 
> ASN is more likely to need 1,208,925,819,614,620,000,000,000 
> right off the bat.  

No, no, no, no, no!

The typical residence can get by with 8 bits of 
subnetting hierarchy, in other words, a /56. 
But the IPv6 architecture really favors giving all
sites, including residences, 16 bits of subnetting
hierarchy, i.e. a /48. A typical ISP will receive
32 bits of subnetting hierarchy, i.e. a /32.

You calculate the number of bits by starting from
the premise that a subnet is assigned a /64 prefix.

There is no point in calculating the number of 
addresses in a prefix since this number is meaningless
in IPv6. For instance, a subnet may have hosts whose
address is randomly assigned and the /64 prefix allows
for this with an extremely low risk of address collisions.

The IPv6 address budget concerns itself with how many
bits of address are used up, not with some theoretical
number of unique combinations can be made with those bits.

Notice that I did not refer to "need". That's because
this is not about need, but about how the IPv6 addressing
architecture was designed to work. It is fundamentally
different from IPv6 and requires a change from IPv4 
thinking, in order to fully understand it.

--Michael Dillon

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