IPCalc is a Perl script some helpful person wrote and put on the internets. The Github and download links are at the bottom of that page.

To find the subnet given IP and mask or CIDR

ipcalc -n 172.16.16.113 255.255.255.252 | grep Network | awk '{print $2}'
172.16.16.112/30

Unless you really love typing you should probably put that in an alias or a script.