Write a bash script to subdivide a given CIDR /24 and smaller subnets into a pre-defined number of smaller subnets.

subnetter.sh

#!/bin/bash

CIDR=""
BASE_IP=""
PARTS=""

validate() {
     if [[ $BASE_IP =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
       :
     else
       echo "IP Address incorrect."
       exit
     fi

    if [[ -z $CIDR ]]; then
		echo "CIDR not entered"
		die
	fi

	if [[ -z $PARTS ]]; then
		echo "PARTS not entered."
		die
	fi
}

sanitise() {
    CIDR=$(echo $1 | awk -F "/" '{print $2}')
    BASE_IP=$(echo $1 | awk -F "/" '{print $1}')
    PARTS=$2
    validate
}

base2() {
    local x=0
    for (( y=$1-1 ; $y > 0; y >>= 1 )) ; do
        let x=$x+1
    done
    echo $x
}

fixip() {
	if [ $IP_OFFSET -ge 256 ]; then
		IP_SET=$(echo $IP_PREFIX | cut -d '.' -f3 )
		ADDITIVE_OFFSET=$(( IP_OFFSET / 256 ))
		NEW_IP_PREFIX=$(echo $(echo $IP_PREFIX | cut -d '.' -f1-2)"."$(( IP_SET + ADDITIVE_OFFSET )))
		NEW_IP_OFFSET=$(( IP_OFFSET % 256 ))
		IP_OFFSET=$NEW_IP_OFFSET
		IP_PREFIX=$NEW_IP_PREFIX
	fi
}

calculate_ips() {
    NUM_BITS=$(( 32 - CIDR ))
    NUM_IPS=$(( 2 ** NUM_BITS ))
    SUBSET=$(( NUM_IPS / PARTS ))
    IP_PREFIX=$(echo $BASE_IP | cut -d '.' -f1-3)
    IP_START_OFFSET=$(echo $BASE_IP | cut -d '.' -f4)

    MINIMUM_IPS_REQURED=$(( SUBSET * 3 + SUBSET ))

    if [ $NUM_IPS -le $MINIMUM_IPS_REQURED ]; then
    	for(( i=0; i < $PARTS; i++)) {
			IP_OFFSET=$(( SUBSET * i ))
       		IP_OFFSET=$(( IP_OFFSET + IP_START_OFFSET ))
        	SUBSET_CIDR=$(( 32 - $(base2 SUBSET ) ))
		HOSTS=$(( ${SUBSET} - 3 ))
		if [ $HOSTS -ge 0 ];then
			fixip
			echo "subnet=${IP_PREFIX}.${IP_OFFSET}/$SUBSET_CIDR network=${IP_PREFIX}.${IP_OFFSET} broadcast=${IP_PREFIX}.$(( $(( IP_OFFSET + SUBSET - 1 )) % 256 )) gateway=${IP_PREFIX}.$(( IP_OFFSET + 1 )) hosts=$(( ${SUBSET} - 3 ))"
		else
			echo "Not Enough IPs"
			exit
		fi
		}
     else
		echo "Not Enough IPs"
     fi
}

main() {
    	sanitise $@
    	calculate_ips
}

usage() { 
	echo "./subnetter.sh <ip> <no of subnets>" 
}

die () {
    usage
    exit 1
}

[ "$#" -ge 1 ] || die 

main $@