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One of my favorites http://www.subnet-calculator.com/
Also a favorite quick subnet cheatsheet https://www.aelius.com/njh/subnet_sheet.html
Any RHEL/CentOS system natively has the ipcalc command to help with subnet calculations.
https://linux.die.net/man/1/ipcalc
Additionally, the following provides an online version of this tool:
http://jodies.de/ipcalc