There are 2^128 IPv6 addresses, which is about 3.4 * 10^38. The surface area of the earth is about 5.1 * 10^14 square meters. So there are about 6.67 * 10^23 addresses per square meter. Interestingly, that’s not very far off from Avogadro’s number—6.02 * 10^23. It’s reasonable that about 10% of the IPv6 address space will be reserved or unusuable, so as an estimate, it’s convenient to remember that Avogadro’s number counts IPv6 addresses per square meter of earth!
And yes, I should be grading right now. How can you tell?