The general form for an IPv6 address is HHHH:HHHH:HHHH:HHHH:HHHH:HHHH:HHHH:HHHH, where HHHH is a 4-byte hexadecimal number, i.e., in the range 0..65535. How many possible IPv6 addresses are there? Find some estimates online for (a) number of grains of sand on Earth, (b) number of neurons in the human brain, (c) number of water molecules on Earth. Give the estimates, and cite your references. In order to make sense of the number of IPv6 addresses there are, it would be good to say "there are roughly as many IPv6 addresses as xxx in yyy". Or even, "there are about 100 as many IPv6 addresses as xxx in yyy". Do some searching, and find some choices for xxx and yyy that provide similar values to the number of possible IPv6 addresses. Cite your references