"In reality the usable space is substantially less; you can start seeing significant collisions once you've filled half the space, but half of an impossibly large number is still impossibly large." Actually, thanks to the birthday paradox, you can expect to see a collision after using the square root of the number of valid identifiers, and the page you linked to even says as much. You've made a common mistake: 2^64 is not half of 2^128.