8
64 bits
Totals bits for IPv6 = 128
IN IPv6 we can find 128bits.
IPv6 uses a 128-bit address space
128
32
The subnet mask itself is an IP Address so it is also 32 bits
we need to borrow 7 bits to subnet 172.16.100.0 to have at least 500 hosts and the subnet mask will be 255.255.254.0
There are 16 bits available; it is up to you how many of those bits you reserve for the subnet, and how many for the individual hosts within each subnet.
A "hextet" in IPv6 consists of 2 bytes, or 4 hexadecimal digits (as in the example in the question), or 16 bits.
16 bits per block
16 bits per block