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")