Just threw together a quick and dirty PHP-based subnet calculator. What sets this one apart from others is that it works with both IPv4 and IPv6 inputs and has subnet splitting capability.
Use the first field to enter a network to get details about. The format is CIDR notation ‘188.8.131.52/22’.
Use the second field to split the first network into smaller networks. The notation is ‘/24’. The network must be smaller than the 1st one obviously. Click any of the smaller subnets to show details.
Source code is on github: