CDK Create Subnets on VPC

Question

I'm trying to create 2 subnets on AWS in CDK.

I originally followed the post here but I ran out of IP Addresses

The error I'm getting is

Resource handler returned message: "The CIDR '12.0.0.0/25' conflicts with another subnet

This error is returned for each of the subnets.

const vpc = new ec2.Vpc(this, name, {
  ipAddresses: IpAddresses.cidr('12.0.0.0/23'),
});    

const publicOneSubnet = new ec2.Subnet(this, 'PublicOneSubnet', {
  availabilityZone: 'eu-west-1a',
  vpcId: vpc.vpcId,
  cidrBlock: '12.0.0.0/25',
})

let publicOneSubnetRouteTable = publicOneSubnet.routeTable;
const publicTwoSubnet = new ec2.Subnet(this, 'PublicTwoSubnet', {
  availabilityZone: 'eu-west-1b',
  vpcId: vpc.vpcId,
  cidrBlock: '12.0.0.128/25'
})

const privateOneSubnet = new ec2.Subnet(this, 'PrivateOneSubnet', {
  availabilityZone: 'eu-west-1a',
  vpcId: vpc.vpcId,
  cidrBlock: '12.0.1.0/25'
})

const privateTwoSubnet = new ec2.Subnet(this, 'PrivateTwoSubnet', {
  availabilityZone: 'eu-west-1b',
  vpcId: vpc.vpcId,
  cidrBlock: '12.0.1.128/25'
})

Could somebody tell me please what I'm doing wrong… this is making me want to cry! As far as I can tell, the 12.0.0.0/23 should mean theres 12.0.1.0-255 and 12.0.0.0-255.

The 12.0.0.0/25 should mean 12.0.0.0-127 and so on, so I'm a bit confused as to how these conflict, I've never done this sort of VPC setup before so apologies if these are stupid questions!

Answer 1

By default your VPC CIDR will be equally divided, 1 public and 1 private subnets will be created per AZ (source).

As you're not specifying anything beside the CIDR on your VPC, maxAzs will be 3 and you'll have a total of 6 subnets created over these availability zones.

If you want to have control over a VPC's subnets, just drop manual subnet creation and use the subnetConfiguration property, while specifying you only want 2 AZs:

const vpc = new ec2.Vpc(this, name, {
  ipAddresses: IpAddresses.cidr('12.0.0.0/23'),
  maxAzs: 2,
  subnetConfiguration: [
    {
      cidrMask: 25,
      name: 'public',
      subnetType: ec2.SubnetType.PUBLIC,
    },
    {
      cidrMask: 25,
      name: 'private',
      subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS,
    }
  ]
})