$ aws ec2 describe-subnets
{
    "Subnets": [
    ...
    ]
}

{
        "AvailabilityZone": "us-east-1d",
        "AvailableIpAddressCount": 251,
        "CidrBlock": "172.30.2.0/24",
        "DefaultForAz": false,
        "MapPublicIpOnLaunch": true,
        "State": "available",
        "SubnetId": "<subnet_id>",
        "VpcId": "<vpc_id>",
        "AssignIpv6AddressOnCreation": false,
        "Ipv6CidrBlockAssociationSet": []
},