Scanning a /64, /56, or /48 isn't really feasible though. A /64 which IIRC is the smallest IPV6 block handed out to home users is 18,446,744,073,709,551,616 address.
So, a IPv6 2^64 subnet is the same as (2^32)x(2^32), which means (4.3B)x(IPv4 Internet). I.e., a single IPv6 subnet can hold the equivalent of four billion (IPv4) Internets.
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A second way of thinking about it:
* Stars in the Milky Way: 400 Billion
* Galaxies in the universe: 2 Trillion
So (4x10^11)x(2x10^12)=8x10^23 stars in the universe.
* Size of IPv6 address space: 3.4x10^38
Find the ratio between addresses and stars:
* 3.4x10^38 / 8x10^23
IPv6 offers about 430 trillion times more addresses than estimated stars in the universe.
From Tom Coffee's presentation "An Enterprise IPv6 Address Planning Case-Study"
On the surface of the Earth (land+water), there are 8.4 IPv4 addresses per km^2. Not counting the oceans, that would be 28 IPv4 addresses per km^2 land.
IPv6 gives 10^17 addresses per mm^2 (yes, square millimeter).
In terms of volume, 10^8 IPv6 addresses per mm^3 throughout the Earth.
