Virtual Private Cloud (VPC) is a Software-Defined Network. You can configure a virtual network on AWS. Subnets are inside of VPC that define network block (Classless Inter Domain Routing (CIDR)) such as 192.168.1.0/24.

Let's create one VPC and two subnets using the following steps:

1. Create a new VPC that has 192.168.0.0/16 CIDR block (IP range: 192.168.0.0 – 192.168.255.255). Then, capture VpcId:

$ aws ec2 create-vpc --cidr-block 192.168.0.0/16
{
    "Vpc": {
        "CidrBlock": "192.168.0.0/16",
        "DhcpOptionsId": "dopt-3d901958",
        "State": "pending",
        "VpcId": "vpc-69cfbd12",
        "InstanceTenancy": "default",
       "Ipv6CidrBlockAssociationSet": [],
        "CidrBlockAssociationSet": [
            {
                "AssociationId": "vpc-cidr-assoc-c35411ae",
                "CidrBlock": "192.168.0.0/16",
                "CidrBlockState": {
                    "State": "associated"
                }
            }
        ],
        "IsDefault": false,
        "Tags": []
    }
}

2. Create the first subnet under the VPC (vpc-69cfbd12) that has 192.168.0.0/24 CIDR block (IP range: 192.168.0.0 – 192.168.0.255) and specify the availability zone as us-east-1a. Then, capture SubnetId:

$ aws ec2 create-subnet --vpc-id vpc-69cfbd12 --cidr-block 192.168.0.0/24 --availability-zone us-east-1a
{
    "Subnet": {
        "AvailabilityZone": "us-east-1a",
        "AvailableIpAddressCount": 251,
        "CidrBlock": "192.168.0.0/24",
        "DefaultForAz": false,
        "MapPublicIpOnLaunch": false,
        "State": "pending",
        "SubnetId": "subnet-6296863f",
        "VpcId": "vpc-69cfbd12",
       "AssignIpv6AddressOnCreation": false,
       "Ipv6CidrBlockAssociationSet": []
    }
}

3. Create the second subnet on us-east-1b, which has 192.168.1.0/24 CIDR block (IP range: 192.168.1.0 – 192.168.1.255). Then, capture SubnetId:

$ aws ec2 create-subnet --vpc-id vpc-69cfbd12 --cidr-block 192.168.1.0/24 --availability-zone us-east-1b
{
    "Subnet": {
        "AvailabilityZone": "us-east-1b",
        "AvailableIpAddressCount": 251,
        "CidrBlock": "192.168.1.0/24",
        "DefaultForAz": false,
        "MapPublicIpOnLaunch": false,
        "State": "pending",
        "SubnetId": "subnet-ce947da9",
        "VpcId": "vpc-69cfbd12",
       "AssignIpv6AddressOnCreation": false,
       "Ipv6CidrBlockAssociationSet": []
    }
}

4. Check the subnet list under VPC (vpc-69cfbd12) using the following command:

$ aws ec2 describe-subnets --filters "Name=vpc-id,Values=vpc-69cfbd12" --query "Subnets[*].{Vpc:VpcId,CIDR:CidrBlock,AZ:AvailabilityZone,Id:SubnetId}" --output=table
---------------------------------------------------------------------
|                          DescribeSubnets                          |
+------------+------------------+-------------------+---------------+
|     AZ     |      CIDR        |       Id          |      Vpc      |
+------------+------------------+-------------------+---------------+
|  us-east-1a|  192.168.0.0/24  |  subnet-6296863f  |  vpc-69cfbd12 |
|  us-east-1b|  192.168.1.0/24  |  subnet-ce947da9  |  vpc-69cfbd12 |
+------------+------------------+-------------------+---------------+

This looks good!