How to Create a Subnet in Terraform: Simple Guide

To create a subnet in Terraform, use the resource "aws_subnet" block specifying the vpc_id, cidr_block, and availability_zone. This defines a subnet within a VPC with the desired IP range and location.

Syntax

The aws_subnet resource creates a subnet in AWS. Key parts include:

• vpc_id: The ID of the VPC where the subnet will be created.
• cidr_block: The IP address range for the subnet in CIDR notation.
• availability_zone: The AWS zone where the subnet resides.

terraform

resource "aws_subnet" "example" {
  vpc_id            = "vpc-12345678"
  cidr_block        = "10.0.1.0/24"
  availability_zone = "us-west-2a"
  tags = {
    Name = "example-subnet"
  }
}

Example

This example creates a subnet in an existing VPC with a specific IP range and availability zone. It also tags the subnet for easy identification.

terraform

provider "aws" {
  region = "us-west-2"
}

resource "aws_vpc" "example_vpc" {
  cidr_block = "10.0.0.0/16"
  tags = {
    Name = "example-vpc"
  }
}

resource "aws_subnet" "example_subnet" {
  vpc_id            = aws_vpc.example_vpc.id
  cidr_block        = "10.0.1.0/24"
  availability_zone = "us-west-2a"
  tags = {
    Name = "example-subnet"
  }
}

Output

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

Common Pitfalls

Common mistakes when creating subnets in Terraform include:

• Using an incorrect vpc_id that does not exist.
• Specifying overlapping cidr_block ranges with other subnets.
• Forgetting to set the availability_zone, which can cause Terraform to pick a default zone unexpectedly.
• Not tagging resources, making management harder.

terraform

/* Wrong: Missing vpc_id */
resource "aws_subnet" "wrong" {
  cidr_block = "10.0.2.0/24"
}

/* Right: Include vpc_id and other required fields */
resource "aws_subnet" "right" {
  vpc_id            = "vpc-12345678"
  cidr_block        = "10.0.2.0/24"
  availability_zone = "us-west-2b"
  tags = {
    Name = "correct-subnet"
  }
}

Quick Reference

Property	Description	Required
vpc_id	ID of the VPC to create the subnet in	Yes
cidr_block	IP range for the subnet in CIDR notation	Yes
availability_zone	AWS zone for the subnet	Recommended
tags	Key-value pairs to label the subnet	No

Key Takeaways

Always specify the correct vpc_id to link the subnet to a VPC.

Use a unique cidr_block that does not overlap with other subnets.

Set availability_zone to control subnet placement.

Tag your subnet resources for easier management.

Validate your Terraform configuration before applying.