KodeKloud Notes

Understanding how AWS handles public IPs and leveraging Elastic IPs can help you maintain consistent connectivity for your applications.

Why Dynamic Public IPs Can Be Problematic

When you launch an EC2 instance in a public subnet, AWS automatically assigns a public IPv4 address (for example, 1.1.1.1). However, this IP is drawn from AWS’s shared pool and is not reserved for your account. Stopping or restarting the instance may result in a new IP, causing:

Downtime if clients have hardcoded the old address
Configuration drift in DNS records or security groups
Operational overhead to track changing IPs

Introducing Elastic IPs

Elastic IP addresses are static IPv4 addresses that you allocate and control within a specific AWS Region. Key benefits include:

Static mapping: The IP stays yours until you explicitly release it
Flexibility: Associate or disassociate the address from EC2 instances or ENIs at any time
High availability: Instantly remap to a standby instance during maintenance or failure

Example: Failover with Elastic IPs

If Server A goes down, simply disassociate its Elastic IP and reassign it to Server B. Clients continue to reach your application at the same address (1.1.1.1), eliminating DNS propagation delays.

The image illustrates an AWS Cloud setup with two servers, Server A and Server B. Server A has an error, while Server B is associated with the IP address 1.1.1.1.

Allocating and Managing Elastic IPs

You can manage Elastic IPs via the AWS Management Console or AWS CLI. Below is a sample CLI workflow.

# Allocate a new Elastic IP in your default VPC
aws ec2 allocate-address --domain vpc


# Associate the Elastic IP with an EC2 instance
aws ec2 associate-address \
  --instance-id i-0123456789abcdef0 \
  --allocation-id eipalloc-12345678


# Disassociate the Elastic IP when needed
aws ec2 disassociate-address --association-id eipassoc-87654321

Note

You can also manage Elastic IPs using AWS SDKs, CloudFormation, or Terraform. Refer to the AWS Elastic IP Documentation for more details.

Elastic IP Pricing

Scenario	Cost
First Elastic IP associated with a running instance	Free
Additional Elastic IPs on the same instance	Charged per hour
Allocated but unattached Elastic IPs	Small hourly fee

Warning

Unattached Elastic IPs incur charges. Always release unused addresses to avoid unexpected costs.

The image illustrates "Elastic IP Pricing," showing a diagram of a microchip with multiple IPs, where additional IPs are charged per hour.

Key Considerations

Elastic IPs are region-specific and cannot be moved across regions.
You can associate them only with EC2 instances or network interfaces (ENIs) in the same region.
Choose between AWS’s public IPv4 pool or bring your own custom IPv4 address block.

Summary

AWS public IPv4 addresses are dynamic and may change on instance stop/start.
Elastic IP addresses provide a static, portable IPv4 address under your control.
Workflow to use an Elastic IP:
1. Allocate it to your AWS account.
2. Associate it with an EC2 instance or ENI.
3. Reassociate as needed during failover or maintenance.

The image is a summary slide explaining the differences between public IPs and Elastic IPs, highlighting that public IPs are not static, while Elastic IPs are static IPv4 addresses. It also describes the process of allocating and associating an Elastic IP with an instance or network interface.

Links and References

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