How subnetting works
A CIDR prefix tells you how many of the 32 bits identify the network. For example, /24 uses 24 network bits and leaves 8 host bits. The number of total IPv4 addresses is 2(32 − prefix).
Typical subnets with prefix /30 or shorter reserve one network address and one broadcast address. The calculator handles the /31 and /32 exceptions separately.
Worked example
For 10.20.30.42/26, the mask is 255.255.255.192. The network address is 10.20.30.0, the broadcast address is 10.20.30.63, and there are 62 traditionally usable host addresses.