Subnet Abacus is an IPv4 Subnet Calculator. In the "Base IP" field, you can specify an IP address (10.0.0.0), an IP address/CIDR combination (10.0.0.0/24), or an IP address/Subnet mask (10.0.0.0 255.255.255.0). You can also the subnet mask using the upper slider. The lower slider is used to divide the upper subnet into smaller subnets.
If a network type other than "Classless" is selected your options will be limited appropriately.
The buttons at the bottom will pop up a seperate window with the appopriate data. Pay attention to the number of subnets or hosts being generated - large numbers make take a long time or crash.