ResolveIPstoSubnets.sh

#!/bin/bash

# Input and output files
INPUT_FILE="hosts.txt"   # File containing hostnames or IPs (one per line)
OUTPUT_FILE="subnets_summary.txt"

# Initialize an empty array to store subnets
declare -A subnets

# Loop through each line in the input file
while IFS= read -r entry || [[ -n "$entry" ]]; do
    # Skip empty lines
    [[ -z "$entry" ]] && continue

    # Perform nslookup
    IP=$(nslookup "$entry" 2>/dev/null | awk '/^Address: / { print $2 }' | tail -n 1)
    
    if [[ -n "$IP" ]]; then
        # Extract the subnet (using /24 by default)
        SUBNET=$(echo "$IP" | awk -F. '{ printf "%s.%s.%s.0/24\n", $1, $2, $3 }')
        subnets["$SUBNET"]=1
        echo "Resolved $entry to $IP, Subnet: $SUBNET"
    else
        echo "Failed to resolve $entry" >&2
    fi
done < "$INPUT_FILE"

# Write unique subnets to the output file
echo "Subnets in scope:" > "$OUTPUT_FILE"
for subnet in "${!subnets[@]}"; do
    echo "$subnet" >> "$OUTPUT_FILE"
done

echo "Subnet summary saved to $OUTPUT_FILE."