Enter an IP address and a subnet mask or CIDR prefix to get the network address, broadcast address, usable host range, and everything else you'd need to plan a subnet.
The network address identifies the subnet itself and the broadcast address is reserved for reaching every host on it — neither is assignable to a device. Everything in between is the usable host range.
The wildcard mask is the inverse of the subnet mask, mainly useful for ACLs on Cisco-style equipment. /31 and /32 are handled as special cases per RFC 3021's point-to-point convention — no network/broadcast reservation applies at those prefix lengths.