Generate Private Ip Cidr Using Shell

Below is the shell script to generate private CIDR range. The CIDR range can be used to usecases lke creating VPC in Amazon AWS.

The script does this for CIDR of /20. You can change the logic accordingly for other address format

##### Generate CIDR #####
value=1
while [ $value -eq 1 ]
do
case $(( RANDOM % 3 )) in
    0)
        a=10
        ;;
    1)
        a=172
        ;;
    2)
        a=192
        ;;
esac
if [ $a -eq 192 ]
then
b=168
c=1
while [ $(($c % 16)) -ne 0 ]
do
        c=$((RANDOM%255))
done
d=0
fi
if [ $a -eq 172 ]
then
b=$[ ( $RANDOM % 31 )  + 16 ]   
c=1
while [ $(($c % 16)) -ne 0 ]
do
        c=$((RANDOM%255))
done
d=0
fi
if [ $a -eq 10 ]
then
b=$[ ( $RANDOM % 255 ) ]
c=1
while [ $(($c % 16)) -ne 0 ]
do
        c=$((RANDOM%255))
done
d=0
fi

##### Generate CIDR END #####