I was once watching a slideshow about the new IPv6, and it mentioned that it is large enough for every grain of sand on earth to be IP addressable.
Is there any grain of truth behind this? (no pun intended)
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Estimating the number of grains of sand on Earth is difficult. This source suggests 7.5x1018 grains (7.5 quintillion), but only includes beaches (deserts, under-sea sand and other sources not included.) This source suggests 1020 to 1024 grains (up to septillion grains of sand).
The number of addresses IPv6 could possibly address is 2128 (excluding reserved addresses), or about 3.4x1038 (340 decillion). Even if you remove the reserved addresses you're still left with far more IPs than grains.
In fact, assuming the most number of grains of sand - around 1024 - 294 femtopercent (yes, femto, 10^-15) would be used if every grain were allocated an IP. You could allocate 340 billion planets with the same number of grains of sand before you even came close to filling up the address space. After all that, you'd still have 2.8x10^35 (280 decillion) addresses free.