This assumes you have already configured the aws command line tool. If not, check out my article on Getting Started with the AWS CLI.

The aws ec2 describe-vpcs command can be used to list your EC2 Virtual Private Clouds (VPC). Something like this should be returned. In this example, the VPC has CIDR 172.31.0.0/20 CIDR.

Tip

Your Virtual Private Cloud (VPC) will be associated with a CIDR block (e.g. 172.31.0.0/16).  In this example, the first available private IP would be 172.31.0.1 and the last available private IP would be 172.31.255.254. Typically, there are 6 subnets in the Virtual Private Cloud (VPC), perhaps something like this.

• 172.31.0.0/20 - Availability Zone us-east-1a
• 172.31.16.0/20 - Availability Zone us-east-1b
• 172.31.32.0/20 - Availability Zone us-east-1c
• 172.31.48.0/20 - Availability Zone us-east-1d
• 172.31.64.0/20 - Availability Zone us-east-1d
• 172.31.80.0/20 - Availability Zone us-east-1f

~]$ aws ec2 describe-vpcs
{
    "Vpcs": [
        {
            "CidrBlock": "172.31.0.0/16",
            "DhcpOptionsId": "dopt-017f01234abcd2fc9",
            "State": "available",
            "VpcId": "vpc-014d2fc1234abcd01",
            "OwnerId": "713541234252",
            "InstanceTenancy": "default",
            "CidrBlockAssociationSet": [
                {
                    "AssociationId": "vpc-cidr-assoc-0ea471234abcddd9f",
                    "CidrBlock": "172.31.0.0/16",
                    "CidrBlockState": {
                        "State": "associated"
                    }
                }
            ],
            "IsDefault": true,
            "Tags": [
                {
                    "Key": "Name",
                    "Value": "default"
                }
            ]
        }
    ]
}

And here is an example of how to limit the output using the --query option.

aws ec2 describe-vpcs --query 'Vpcs[?CidrBlock==`172.31.0.0/16`]'

The aws ec2 describe-subnets command can be used to list the EC2 subnets in your Virtual Private Clouds (VPC).

aws ec2 describe-subnets

Or, the --filter option can be used to list the Subnets in a specific VPC.

aws ec2 describe-subnets --filter "Name=vpc-id,Values=vpc-014d2fc1234abcd01"

In all liklihood, there are probably 3 (or more) subnets for these CIDRs.

• 172.31.0.0/20 (first available IP 172.31.0.1, last available IP 172.31.15.254)
• 172.31.16.0/20 (first available IP 172.16.0.1, last available IP 172.31.31.254)
• 172.31.32.0/20 (first available IP 172.32.0.1, last available IP 172.31.47.254)

{
    "Subnets": [
        {
            "AvailabilityZone": "us-east-1d",
            "AvailabilityZoneId": "use1-az6",
            "AvailableIpAddressCount": 4091,
            "CidrBlock": "172.31.0.0/20",
            "DefaultForAz": true,
            "MapPublicIpOnLaunch": true,
            "MapCustomerOwnedIpOnLaunch": false,
            "State": "available",
            "SubnetId": "subnet-03f11411234f6abcd",
            "VpcId": "vpc-014d2123433abcd01",
            "OwnerId": "713512344252",
            "AssignIpv6AddressOnCreation": false,
            "Ipv6CidrBlockAssociationSet": [],
            "SubnetArn": "arn:aws:ec2:us-east-1:711234074252:subnet/subnet-031234abcd0f6cdbc",
            "EnableDns64": false,
            "Ipv6Native": false,
            "PrivateDnsNameOptionsOnLaunch": {
                "HostnameType": "ip-name",
                "EnableResourceNameDnsARecord": false,
                "EnableResourceNameDnsAAAARecord": false
            }
        }
    ]
}