Organizing VPC subnets with multiple IPv4 CIDR blocks

Question

Pulumi · Accepted Answer

Creating a Virtual Private Cloud (VPC) with multiple subnets each with its own IPv4 CIDR block is an essential task for network segregation and management of cloud resources. In this Pulumi TypeScript program, we will define a VPC and then divide it into multiple subnets, specifying a unique IPv4 CIDR block for each.

To accomplish this, we will use the `aws.ec2.Vpc` resource to create a new VPC, and multiple `aws.ec2.Subnet` resources to create the subnets within the newly created VPC. The `aws.ec2.Vpc` resource allows us to define a VPC with a specified CIDR block, and within this VPC, we can create multiple subnets using `aws.ec2.Subnet`, each with a portion of the VPC's CIDR block specified as the subnet's CIDR.

Pulumi's infrastructure as code approach allows us to define these resources in a clear, declarative way, which ensures that our infrastructure is reproducible and version-controllable.

### Detailed Breakdown of the Program:
- We import the necessary Pulumi and AWS SDK components.
- We initialize a new VPC using the `aws.ec2.Vpc` resource.
- We create multiple subnets using the `aws.ec2.Subnet` resource within the VPC, providing unique CIDR blocks for each subnet.
- We export the VPC and subnet IDs for use in other parts of our infrastructure or for reference.

Let's start by writing the Pulumi program.

```typescript
import * as pulumi from "@pulumi/pulumi";
import * as aws from "@pulumi/aws";

// Create a VPC with a specified CIDR block
const vpc = new aws.ec2.Vpc("customVpc", {
    cidrBlock: "10.0.0.0/16", // The entire range of addresses for this VPC.
    enableDnsHostnames: true, // Allows instances in the VPC to have a DNS hostname.
    enableDnsSupport: true,   // Allows DNS resolution within the VPC.
    tags: {
        Name: "custom-vpc",   // Naming tag for easy identification of the VPC.
    },
});

// Define array of CIDR blocks for subnets
const subnetCidrBlocks = [
    "10.0.1.0/24",
    "10.0.2.0/24",
    "10.0.3.0/24", // Add as many as needed for your infrastructure
    // ... extend the list as per your requirements
];

// Create multiple subnets with the CIDR blocks specified
const subnets = subnetCidrBlocks.map((cidrBlock, index) => 
    new aws.ec2.Subnet(`subnet-${index}`, {
        vpcId: vpc.id,              // Associates subnet with the VPC created above.
        cidrBlock,                  // CIDR block for the subnet.
        tags: {
            Name: `subnet-${index}` // Naming tag for easy identification of the subnet.
        },
    })
);

// Export the IDs of the VPC and Subnets
export const vpcId = vpc.id;
export const subnetIds = subnets.map(subnet => subnet.id);
```

In this example:
- We created a VPC named `customVpc` with a CIDR block `10.0.0.0/16`.
- We then defined an array `subnetCidrBlocks` that contains the CIDR blocks for each of the subnets we want to create within this VPC.
- We mapped over this array to create `aws.ec2.Subnet` resources for each CIDR block.
- Each subnet is tagged with its name and index for easy identification.
- Finally, we export the VPC and subnet IDs so they can be used elsewhere or queried from the Pulumi CLI.

By running this program with Pulumi, the defined VPC and subnets will be created in your AWS account. With Pulumi's real-time preview feature, you will be able to see the proposed changes before they are applied, providing you with full control and visibility into your infrastructure.