This is a simple and useful app for calulating the range of the host part of a subnet mask within an IPv4 network
You can (optionally) enter your client IP. Jumping between the fields of the 4 octetts can be done using the decimal seperator key for quick input. Then use the slider to set the subnet bits of your mask. The app shows you then details of your network, like client count, client range, network address, broadcast address and wildcard mask.
There's also an information screen that shows you your IP addresses (LAN and Internet).
A useful tool for every network engineer.