How to fix Error: Error creating subnet: InvalidSubnet.Range in Terraform

# View your Terraform state or configuration
terraform state show aws_vpc.main
terraform state show aws_subnet.example

# Or check directly in AWS
aws ec2 describe-vpcs --query 'Vpcs[*].[VpcId,CidrBlock]' --output table

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"  # Your VPC range
}

resource "aws_subnet" "example" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"  # Must be within 10.0.0.0/16
}

aws ec2 describe-subnets --filters "Name=vpc-id,Values=vpc-xxxxx" \
  --query 'Subnets[*].[SubnetId,CidrBlock,AvailabilityZone]' --output table

terraform state list | grep aws_subnet
terraform state show aws_subnet.subnet1
terraform state show aws_subnet.subnet2

# Define VPC with /16 CIDR block
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

# Create subnets safely using cidrsubnet
resource "aws_subnet" "public_1" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 2, 0)  # 10.0.0.0/18
  availability_zone = "us-east-1a"
}

resource "aws_subnet" "public_2" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 2, 1)  # 10.0.64.0/18
  availability_zone = "us-east-1b"
}

resource "aws_subnet" "private_1" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 2, 2)  # 10.0.128.0/18
  availability_zone = "us-east-1a"
}

# If this fails with too many subnets:
cidr_block = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index)  # /24

# Try a larger block size:
cidr_block = cidrsubnet(aws_vpc.main.cidr_block, 4, count.index)  # /20

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_vpc_ipv4_cidr_block_association" "secondary" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.2.0.0/16"
}

# Error: Secondary CIDR not ready yet!
resource "aws_subnet" "secondary_subnet" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.2.0.0/24"
}

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_vpc_ipv4_cidr_block_association" "secondary" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.2.0.0/16"
}

# Explicitly depend on secondary CIDR association
resource "aws_subnet" "secondary_subnet" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.2.0.0/24"
  depends_on        = [aws_vpc_ipv4_cidr_block_association.secondary]
}

# First apply: Creates VPC and associates secondary CIDR
terraform apply

# Second apply: Creates subnets once CIDR is fully ready
terraform apply

terraform plan -out=tfplan

# Use AWS CLI to validate
aws ec2 describe-vpcs --vpc-ids vpc-12345678 \
  --query 'Vpcs[0].[CidrBlock,Ipv6CidrBlockAssociationSet]'

cidr_block = "10.0.1.0/24"  # CORRECT

# Missing leading zeros
cidr_block = "10.0.1.0/024"      # WRONG (invalid prefix)

# Wrong notation
cidr_block = "10.0.1.0-10.0.1.255"  # WRONG (use CIDR notation)

# Subnet with host bits set
cidr_block = "10.0.1.128/25"
# This is 10.0.1.128 to 10.0.1.255, but the /25 prefix expects the network
# to end at a /25 boundary. Use 10.0.1.0/25 or 10.0.1.128/25 correctly.

# Using public IP ranges for private subnets
cidr_block = "8.8.8.0/24"  # Public range - won't work for private VPC

# BAD: May cause validation errors during planning
variable "subnet_cidr" {
  type = string
}

resource "aws_subnet" "bad" {
  vpc_id     = aws_vpc.main.id
  cidr_block = var.subnet_cidr  # Terraform can't validate before applying
}

# GOOD: Validate with Terraform functions
resource "aws_subnet" "good" {
  vpc_id     = aws_vpc.main.id
  cidr_block = cidrsubnet(aws_vpc.main.cidr_block, 2, var.subnet_index)
}

# Step 1: Enable IPv6 on VPC
resource "aws_vpc" "main" {
  cidr_block           = "10.0.0.0/16"
  assign_ipv6_cidr_block = true
}

# Step 2: Wait for IPv6 CIDR association
resource "aws_vpc_ipv6_cidr_block_association" "main" {
  vpc_id            = aws_vpc.main.id
  amazon_provided_ipv6_cidr_block = true
}

# Step 3: Create subnet with IPv6
resource "aws_subnet" "example" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.1.0/24"

  # IPv6 CIDR block from VPC
  ipv6_cidr_block = cidrsubnet(aws_vpc.main.ipv6_cidr_block, 8, 1)

  assign_ipv6_address_on_creation = true

  depends_on = [aws_vpc_ipv6_cidr_block_association.main]
}

# WRONG: Empty IPv6 CIDR will cause error
resource "aws_subnet" "bad" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.1.0/24"
  ipv6_cidr_block   = ""  # Error!
}

# Either set a valid block or omit the argument entirely

# Validate syntax
terraform validate

# Plan to see what will be created
terraform plan

# Apply the changes
terraform apply

# List all subnets in your VPC
aws ec2 describe-subnets --filters "Name=vpc-id,Values=vpc-xxxxx" \
  --query 'Subnets[*].[SubnetId,CidrBlock,AvailabilityZone]' \
  --output table

# Or check a specific subnet
aws ec2 describe-subnets --subnet-ids subnet-xxxxx \
  --query 'Subnets[0]'