amazon web services - How to calculate total ip's of a subnet in AWS

Question

Recently we got an issue of IP running out in VPC due to huge IP's consumed by ENI. I need to write a script in boto3, to trigger an alert if total ip usage increases around 80% or something.

Hence I need to know the Total ip's allocated in a subnet in VPC. I need the total count of used IP's and free IP's.

Please share the boto3 commands for doing it.

Answer 1

The awscli ec2 describe-subnets call will actually return you the number of unused private IPv4 addresses in the subnet. The IPv4 addresses for any stopped instances are considered unavailable.

For example:

aws ec2 describe-subnets 
    --subnet-ids subnet-c0c1a23a 
    --query "Subnets[0].AvailableIpAddressCount"

Sample output:

249

To calculate the total number of usable IPs in the subnet 10.0.0.0/24 or more generally a /N:

10.0.0.0/24 => 2**(32-24) - 5
10.0.0.0/N  => 2**(32-N) - 5

Note that you subtract 5 because the first four IP addresses and the last IP address in each subnet CIDR block are reserved by AWS, and cannot be assigned to an instance.

And, for good measure, a Python script:

import boto3

ec2 = boto3.resource('ec2')

# Use this for specific subnets
# filters = [{'Name':'subnet-id', 'Values':['subnet-c0c1a23a']}]
# subnets = ec2.subnets.filter(Filters=filters)

# Use this for all subnets
subnets = ec2.subnets.all()

for subnet in list(subnets):
    free_ips = subnet.available_ip_address_count
    n = int(subnet.cidr_block.split('/')[1])
    cidr_ips = 2**(32-n)
    used_ips = cidr_ips - free_ips
    print('{:s}: cidr={:d}, aws used=5, you used={:d}, free={:d}'.
        format(subnet.id, cidr_ips, used_ips - 5, free_ips))

Sample output:

subnet-1eb2e345: cidr=256, free=251, aws used=5, you used=0
subnet-c0c1a23a: cidr=256, free=249, aws used=5, you used=2