I always use IP Calculator.

http://jodies.de/ipcalc

ipcalc takes an IP address and netmask and calculates the resulting broadcast, network, Cisco wildcard mask, and host range.

Enter your netmask(s) in CIDR notation (/25) or dotted decimals (255.255.255.0).

The wildcard is the inverse netmask as used for access control lists in Cisco routers.

http://cidr.xyz/

http://mxtoolbox.com/subnetcalculator.aspx