Large, sparse subnets are nice for their security benefits. For a 16-bit network, you can enumerate all active public servers by exhaustively port-scanning it;

Large, sparse subnets are nice for their security benefits. For a 16-bit network, you can enumerate all active public servers by exhaustively port-scanning it; it takes something like a few hundred gigabytes of traffic, which is nothing these days. For a 64-bit network, it takes quadrillions of gigabytes of traffic and just isn't feasible. 64 bits is enough space to fit a small public key, which v6 uses to secure neighbor discovery. There are anonymity benefits too: privacy extensions wouldn't work as well on smaller subnets. As an added bonus, having extra bits to spare is useful if it ever turns out that we need them. If we run out of space in 2000::/3 then we can start over in one of the five other unused /3s using tighter allocation policies. L3 protocols are incredibly hard to deploy and it would really suck to deploy a bigger one only to have to deploy another, even bigger one again soon after. I don't think there's a good reason to give up all of that. Smaller addresses break compatibility with v4 just as thoroughly as bigger ones do, so it wouldn't even help deployment much.