About this interactive
An IPv6 address has no subnet mask: a prefix length says how many of its first bits are fixed. On a LAN that is almost always /64, so the first four hextets are the network and the last four are the interface ID, the host's own part. An organisation is usually given a shorter prefix, often a /48 or a /56, and the bits between that prefix and /64 are its subnet ID: n bits number 2^n subnets, and a /48 leaves a whole hextet of them. Because each hex digit is a nibble of 4 bits, prefixes that end on a nibble (/52, /56, /60) are easy to read.
Numbering subnets is counting in hex. The first subnet is all zeros, so the 40th subnet is number 39, which is 0010 0111 in binary and 27 in hex: its network is the /48 with :27 in the fourth hextet. Hosts do not come into it: a /64 holds far more hosts than any LAN, so you plan the number of networks, not their size.
Seven of the twenty-two questions are drawn each run. The addresses use the IPv6 documentation prefixes (2001:db8::/32 and 3fff::/20). Anything you miss shows the answer and how to work it out.
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