ip2cidr is a small and simple program for converting an ip address (mask) into a CIDR, or returning an already valid CIDR.

Edit config.mk to match your local setup (ip2cidr is installed into the /usr/local namespace by default), then simply enter the following command to install (if necessary as root):

make clean install

Allow user to enter a subnet mask as either a CIDR or as an ip address:

read -p "enter subnet mask: " output; output=$(ip2cidr "$output")