This is an amazingly handy IP Subnet Calculator: BitCricket IP Subnet Calculator for IPv4/v6.
Simply input an IP address and choose one of the following: number of subnet bits, the maximum number of desired subnets, the number of host bits, or the maximum number of desired hosts. Our calculator figures out the all the rest, giving you a bit-by-bit address visualization and generating a complete table of subnets. Quick. Easy. Painless.