AWS-Terraform Public, Private IPs, and Elastic IPs

Public and Private IPs

IP Versions:

• IPv4: Common format, four numbers separated by dots, provides 3.7 billion unique addresses.
• IPv6: Less common, longer alphanumeric string, supports more addresses, used for IoT.

Public IP:

• Used for identifying machines on the internet, must be unique globally.
• Allows communication over the internet.
• Associated with instances like EC2 when they need direct internet access.

Private IP:

• Used within private networks (e.g. within a company).
• Allows communication only within the network, not accessible from the internet.
• Can have identical IPs across different private networks.

NAT Devices and Gateways:

• Machines in a private network connect to the internet through NAT devices and internet gateways acting as proxies.

Elastic IPs:

• Public IPv4 addresses that you can attach to instances.
• Provide a fixed public IP to mask instances or software failures.
• Limited to five per AWS account by default, considered poor architectural practice due to potential scalability issues.

Best Practices:

• Avoid using Elastic IPs where possible; prefer DNS names or load balancers for scalable and manageable solutions.
• DNS (e.g., Route 53) provides scalable and controlled domain name resolution.

Behavior of EC2 Instances:

• By default, EC2 instances have a private IP for internal AWS network and a public IP for internet access.
• Public IPs may change if instances are stopped and started.
• SSH into EC2 instances typically uses the public IP unless connected via VPN.

Public and Private IP Usage:

• Public IPv4 is used to SSH into the instance from the internet.
• Once logged in, the private IP can be used for internal communication.
• SSH via private IP does not work unless connected to the same private network (e.g. via VPN).

Behavior on Start/Stop:

• Stopping and starting an instance changes its public IPv4 address.
• The private IP remains the same across starts and stops.
• Rebooting does not change the public IP, but stopping and starting does.

Elastic IPs:

• Elastic IPs provide a fixed public IPv4 address that persists across instance stop/start cycles.
• An Elastic IP can be allocated from Amazon’s pool and associated with an instance.
• There is a small hourly charge for Elastic IPs if they are not in use.

Cost Considerations:

• Using Elastic IPs incurs a cost, of around $0.005 per hour or $3.50 per month.
• AWS offers 750 hours per month of free public IPv4 addresses, so managing instances and Elastic IPs efficiently helps avoid unnecessary costs.

Elastic IP Association:

• An Elastic IP can be associated with an instance, ensuring it retains the same public IPv4 address even when stopped and started.
• Disassociating and releasing the Elastic IP prevents ongoing charges.

Hands-On Example:

• Demonstrated stopping, starting, and associating Elastic IPs with an instance.

• Showed that Elastic IP remains consistent even after instance stop/start.
• Go to Elastic IP address >> Allocate Elastic IP address

Once the Elastic IP address is created attach it to the Instance >> Associate Elastic IP address

Even the instance is stopped and started again the IP will not change

• Highlighted the importance of releasing unused Elastic IPs to avoid charges.

How to Attach Elastic IP to EC2 Instance using Terraform?

#Creating EC2 

resource "aws_instance" "server" {
  ami           = data.aws_ami.aws_ami.id
  instance_type = "t2.micro"

  tags = {
    Name = "fluffy-server"
  }
}

# Data block 

data "aws_ami" "aws_ami" {
  most_recent = true
  owners      = ["amazon"]  # Specify the owner (e.g., "amazon" for official AMIs)
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*"]  # Replace with the desired AMI name
  }
}

#Creating elastic IP

resource "aws_eip" "lb" {
  vpc = true 

  tags = {
    Name = "fluffy-eip"
  }
}

#Attaching Elatic IP to Instance 

resource "aws_eip_association" "eip_assoc" {
  instance_id   = aws_instance.server.id
  allocation_id = aws_eip.lb.id
}

If you found this guide helpful then do click on 👏 the button.

Follow for more Learning like this 😊

If there’s a specific topic you’re curious about, feel free to drop a personal note or comment. I’m here to help you explore whatever interests you!

Thanks for spending your valuable time learning to enhance your knowledge!