Don't think in terms of addresses. In a hierarchically-allocated and aggregated space like IP addresses, there's a massive difference between 64 bits of address space and 2^64 usable addresses.

By the time you've gone through six levels of allocations (IANA -> RIR -> ISP -> Customer -> Network -> Device) the vast majority of the total address space will be unused -- or rather: used for the purpose of aggregation, but not used for end machines. A 64-bit address space might only reasonably be able to handle something like 2^40-2^48 devices in total, which isn't that much more than we already have today.

> Unless you're expecting consumers to start having ~2^64 devices

You'd definitely need something longer than 64 bits long, long before this point.

If ISPs are allocating /64s or more to end consumers, how is this different from a 64 bit address space where they handed out a single address to each consumer?

In terms of rate of consumption of the overall address space, it's not. But consumers have started to have more than one device at home -- in fact almost everybody has an entire network with multiple machines these days, so a single address isn't sufficient.

v6 has enough space to give everybody multiple networks, not just multiple addresses (and those networks are always big enough for however many devices you end up attaching to them).

To illustrate the scale, 2^32:

* is roughly equal to 4 300 000 000 (4e9, 4B)

* it is also the maximum number of IPv4 addresses

Going back to math class:

* 2^64 = 2^(32+32) = 2^32 * 2^32 = (4B) * (IPv4 Internet)

So a single /64 IPv6 subnet can hold four billion IPv4 Internets worth of systems.

The point was whether 128 bit addresses are actually required - someone was claiming that 64 bit addresses would have also been exhausted, and I pointed out that the current strategy for handing out IPv6 is anyway using up the address space at a similar pace to the situation where we had a 64 bit address and were handing out /64 addresses to individual consumers. There were even some comments claiming certain ISPs are handing out /56 spaces to end consumers.

Ethernet MACs have 48-bit addresses, and I don't think we've run out of those, and I also don't think is anyone is even worrying about running out of those, so 64-bit IP addresses probably would have been sufficient.

But given all the drama with IPv6, I think it was reasonable to go with 128-bit addresses just so we don't have to go through all of this again.

On a related note, messing up the assignment of IPv6 address was actually accounted for. If you look at any unicast address that's currently live, you'll see that it begins with a "2". This is because addresses are only being assigned from 2000::/3:

* https://www.iana.org/assignments/ipv6-address-space/ipv6-add...

If, for some reason, we realize there's a mistake in how things were done with that space, it will be declared legacy/deprecated, and IANA will start over with addresses from 4000::/3. If that is screwed up, then 6000::/3 will be next, then 8000::/3, a000::/3, and finally c000::/3.

So the Internet community has six tries to get IPv6 addressing correct.