Provision an AWS VPC with Terraform

A well-structured VPC is the foundation of secure cloud infrastructure. Terraform allows you to define the entire network topology — subnets, routing, gateways, and security groups — as version-controlled code that can be recreated identically across environments.

When to Use This

• You need repeatable, version-controlled network infrastructure across dev/staging/prod
• Applications require both public-facing and internal-only resources
• You want to enforce network segmentation between different tiers

Prerequisites

• AWS CLI configured with appropriate credentials
• Terraform 1.5+ installed

Solution

1. VPC and Subnets

# vpc.tf
locals {
  azs = ["us-east-1a", "us-east-1b", "us-east-1c"]
}

resource "aws_vpc" "main" {
  cidr_block           = "10.0.0.0/16"
  enable_dns_hostnames = true
  enable_dns_support   = true

  tags = {
    Name = "production-vpc"
  }
}

resource "aws_subnet" "public" {
  count                   = length(local.azs)
  vpc_id                  = aws_vpc.main.id
  cidr_block              = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index)
  availability_zone       = local.azs[count.index]
  map_public_ip_on_launch = true

  tags = {
    Name = "public-subnet-${count.index + 1}"
    Type = "public"
  }
}

resource "aws_subnet" "private" {
  count             = length(local.azs)
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index + 100)
  availability_zone = local.azs[count.index]

  tags = {
    Name = "private-subnet-${count.index + 1}"
    Type = "private"
  }
}

2. Internet and NAT Gateways

# gateways.tf
resource "aws_internet_gateway" "main" {
  vpc_id = aws_vpc.main.id
  tags   = { Name = "main-igw" }
}

resource "aws_eip" "nat" {
  count  = length(local.azs)
  domain = "vpc"
  tags   = { Name = "nat-eip-${count.index + 1}" }
}

resource "aws_nat_gateway" "main" {
  count         = length(local.azs)
  allocation_id = aws_eip.nat[count.index].id
  subnet_id     = aws_subnet.public[count.index].id
  tags          = { Name = "nat-gateway-${count.index + 1}" }
}

3. Route Tables

# routing.tf
resource "aws_route_table" "public" {
  vpc_id = aws_vpc.main.id

  route {
    cidr_block = "0.0.0.0/0"
    gateway_id = aws_internet_gateway.main.id
  }

  tags = { Name = "public-rt" }
}

resource "aws_route_table_association" "public" {
  count          = length(local.azs)
  subnet_id      = aws_subnet.public[count.index].id
  route_table_id = aws_route_table.public.id
}

resource "aws_route_table" "private" {
  count  = length(local.azs)
  vpc_id = aws_vpc.main.id

  route {
    cidr_block     = "0.0.0.0/0"
    nat_gateway_id = aws_nat_gateway.main[count.index].id
  }

  tags = { Name = "private-rt-${count.index + 1}" }
}

resource "aws_route_table_association" "private" {
  count          = length(local.azs)
  subnet_id      = aws_subnet.private[count.index].id
  route_table_id = aws_route_table.private[count.index].id
}

4. Security Groups

# security.tf
resource "aws_security_group" "web" {
  name_prefix = "web-"
  vpc_id      = aws_vpc.main.id
  description = "Allow HTTP and HTTPS traffic"

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  tags = { Name = "web-sg" }
}

resource "aws_security_group" "database" {
  name_prefix = "db-"
  vpc_id      = aws_vpc.main.id
  description = "Allow database access from web tier only"

  ingress {
    from_port       = 5432
    to_port         = 5432
    protocol        = "tcp"
    security_groups = [aws_security_group.web.id]
  }

  tags = { Name = "database-sg" }
}

How It Works

1. VPC defines the private IP address space for all resources
2. Public Subnets route traffic through the Internet Gateway for external access
3. Private Subnets route outbound traffic through NAT Gateways for security
4. Route Tables control traffic direction per subnet
5. Security Groups act as stateful firewalls at the instance level

Production Considerations

• Use Terraform workspaces or separate state files for each environment
• Enable VPC Flow Logs to CloudWatch for network traffic auditing
• Place databases and internal services in private subnets only
• Use AWS Network Firewall or Security Groups for defense in depth

Common Mistakes

• Forgetting to enable map_public_ip_on_launch for public subnet instances
• Placing NAT Gateways in private subnets instead of public subnets
• Using overly permissive CIDR blocks like 0.0.0.0/0 in security group ingress

FAQ

Q: Should I use one NAT Gateway or one per AZ? A: One per AZ eliminates a single point of failure and avoids cross-AZ data transfer costs. For cost-sensitive dev environments, one NAT Gateway is acceptable.

Q: How do I peer this VPC with another? A: Use aws_vpc_peering_connection and add routes in both VPCs pointing to the peered CIDR block.

Q: Can I import an existing VPC into Terraform? A: Yes. Use terraform import aws_vpc.main <vpc-id> and then write the matching configuration.