8
64 bits
In IPv6, the multicast address subnet ID is 64 bits long. This is part of the overall 128-bit address structure, where the first 8 bits are reserved for the multicast prefix (FF00::/8), followed by 16 bits for flags and scope, and the remaining 64 bits represent the group ID. Thus, the group ID allows for a large number of multicast groups within the IPv6 space.
A subnet with a prefix of /100 indicates that the first 100 bits of the IP address are fixed for the network portion. However, since IPv4 addresses are only 32 bits long, a subnet mask of /100 is not valid in this context. In IPv6, which allows for longer prefixes, a /100 subnet would have 100 bits dedicated to the network, leaving 28 bits for host addresses.
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.