audit how many IPs are in use by a list of subnets

ip_audit.py

"""subnet IP usage audit"""

# stdlib
from ipaddress import IPv4Network

# 3rd party
import boto3

# local

subnets = ['subnet-xxx']

def main():
    ec2 = boto3.client('ec2', region_name='us-east-1')
    for subnet in subnets:
        result = ec2.describe_subnets(SubnetIds=[subnet])['Subnets'][0]
        network = IPv4Network(result.get('CidrBlock'))
        # aws reserves 3 addresses per subnet CIDR: 
        # https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html#VPC_Sizing
        max_possible_addresses = len(list(network.hosts())) - 3
        available_ips = result.get('AvailableIpAddressCount')
        percent_used = (available_ips / max_possible_addresses) * 100
        fraction = f"{available_ips}/{max_possible_addresses}"
        msg = f"{subnet} - {result.get('CidrBlock'):<18} {fraction:<7} {percent_used:.1f}%"
        print(msg)


if __name__ == '__main__':
    main()