ipcalc
http://tuxgraphics.org/toolbox/network_address_calculator_add.html
http://www.subnetonline.com/pages/subnet-calculators/ip-subnet-calculator.php
http://jodies.de/ipcalc?host=10.61.42.1&mask1=21&mask2=
http://ipcalc.nmonitoring.com/
https://www.iplocation.net/subnet-calculator
https://mxtoolbox.com/subnetcalculator.aspx
http://www.solarwinds.com/free-tools/advanced-subnet-calculator
http://www.ipaddressguide.com/cidr