Getting Elastic IP Associated with a Public Subnet

Oct 13, 2025 by wbrum.org 84 views

Introduction

Amazon Web Services (AWS) provides a wide range of services to help users manage and deploy their applications in the cloud. One of the key services is Elastic IP (EIP), which allows users to associate a static IP address with their instance. However, when it comes to getting the Elastic IP associated with a Public Subnet, users may encounter some challenges. In this article, we will explore a method to get the Elastic IP associated with a Public Subnet using AWS Cloud Development Kit (CDK).

What is AWS CDK?

AWS CDK is an open-source framework that allows developers to define cloud infrastructure in code. It provides a set of high-level APIs and a domain-specific language (DSL) to create cloud infrastructure. With AWS CDK, developers can define their infrastructure as code, which makes it easier to manage and maintain their cloud resources.

What is a Public Subnet?

A Public Subnet is a subnet that is connected to the internet. It is used to deploy instances that need to be accessible from the internet. When creating a Public Subnet, users can specify whether to map the public IP address on launch. If map_public_ip_on_launch is set to True, the instance will be assigned a public IP address.

Getting Elastic IP Associated with a Public Subnet

To get the Elastic IP associated with a Public Subnet, we need to use the aws_ec2 module in AWS CDK. Here is an example of how to do it:

import aws_cdk as cdk
from aws_cdk import aws_ec2 as ec2
class MyStack(cdk.Stack):
def init(self, scope: cdk.Construct, id: str, **kwargs) -> None:
super().init(scope, id, **kwargs)
    # Create a VPC
    vpc = ec2.Vpc(self, &quot;VPC&quot;)

    # Create a Public Subnet
    public_subnet = ec2.Subnet(self, &quot;Public Subnet&quot;,
        vpc=vpc,
        cidr=&quot;10.0.1.0/24&quot;,
        map_public_ip_on_launch=True
    )

    # Get the Elastic IP associated with the Public Subnet
    elastic_ip = ec2.ElasticIp(self, &quot;Elastic IP&quot;,
        vpc=vpc
    )

    # Print the Elastic IP
    print(elastic_ip)

However, as you mentioned, this code will not work because it does not associate the Elastic IP with the Public Subnet. To fix this issue, we need to use the associate_eip method of the ec2.Instance class.

Associating Elastic IP with Public Subnet

Here is the updated code:

import aws_cdk as cdk
from aws_cdk import aws_ec2 as ec2
class MyStack(cdk.Stack):
def init(self, scope: cdk.Construct, id: str, **kwargs) -> None:
super().init(scope, id, **kwargs)
    # Create a VPC
    vpc = ec2.Vpc(self, &quot;VPC&quot;)

    # Create a Public Subnet
    public_subnet = ec2.Subnet(self, &quot;Public Subnet&quot;,
        vpc=vpc,
        cidr=&quot;10.0.1.0/24&quot;,
        map_public_ip_on_launch=True
    )

    # Create an instance in the Public Subnet
    instance = ec2.Instance(self, &quot;Instance&quot;,
        vpc=vpc,
        instance_type=&quot;t2.micro&quot;,
        machine_image=ec2.MachineImage.latest_amazon_linux(
            generation=ec2.AmazonLinuxGeneration.AMAZON_LINUX_2,
            edition=ec2.AmazonLinuxEdition.STANDARD,
            virtualization=ec2.AmazonLinuxVirt.HVM
        ),
        vpc_subnets=ec2.SubnetSelection(subnets=[public_subnet])
    )

    # Associate an Elastic IP with the instance
    elastic_ip = ec2.ElasticIp(self, &quot;Elastic IP&quot;,
        vpc=vpc
    )
    instance.apply_aspect(ec2.Aspect.ElasticIp(elastic_ip))

    # Print the Elastic IP
    print(elastic_ip)

This code creates an instance in the Public Subnet and associates an Elastic IP with the instance. The Elastic IP is then printed to the console.

Conclusion

In this article, we explored a method to get the Elastic IP associated with a Public Subnet using AWS CDK. We created a VPC, a Public Subnet, and an instance in the Public Subnet. We then associated an Elastic IP with the instance and printed the Elastic IP to the console. This code provides a basic example of how to get the Elastic IP associated with a Public Subnet using AWS CDK.

Future Work

In the future, we can improve this code by adding more features, such as:

Creating multiple instances in the Public Subnet and associating an Elastic IP with each instance.
Creating a load balancer in the Public Subnet and associating an Elastic IP with the load balancer.
Creating a security group in the Public Subnet and associating an Elastic IP with the security group.

By adding these features, we can make the code more robust and useful for users who need to manage their cloud resources.

References

AWS CDK Documentation: https://docs.aws.amazon.com/cdk/latest/guide/getting_started.html
AWS EC2 Documentation: https://docs.aws.amazon.com/ec2/latest/UserGuide/using-eni.html
AWS Elastic IP Documentation: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip.html
AWS CDK: A Method to Get Elastic IPs Associated with a Public Subnet - Q&A ====================================================================

Introduction

In our previous article, we explored a method to get the Elastic IP associated with a Public Subnet using AWS Cloud Development Kit (CDK). We created a VPC, a Public Subnet, and an instance in the Public Subnet. We then associated an Elastic IP with the instance and printed the Elastic IP to the console. In this article, we will answer some frequently asked questions (FAQs) related to getting Elastic IPs associated with Public Subnets using AWS CDK.

Q: What is the difference between a Public Subnet and a Private Subnet?

A: A Public Subnet is a subnet that is connected to the internet. It is used to deploy instances that need to be accessible from the internet. A Private Subnet, on the other hand, is a subnet that is not connected to the internet. It is used to deploy instances that do not need to be accessible from the internet.

Q: How do I create a Public Subnet using AWS CDK?

A: To create a Public Subnet using AWS CDK, you can use the ec2.Subnet class. Here is an example of how to create a Public Subnet:

import aws_cdk as cdk
from aws_cdk import aws_ec2 as ec2
class MyStack(cdk.Stack):
def init(self, scope: cdk.Construct, id: str, **kwargs) -> None:
super().init(scope, id, **kwargs)
    # Create a VPC
    vpc = ec2.Vpc(self, &quot;VPC&quot;)

    # Create a Public Subnet
    public_subnet = ec2.Subnet(self, &quot;Public Subnet&quot;,
        vpc=vpc,
        cidr=&quot;10.0.1.0/24&quot;,
        map_public_ip_on_launch=True
    )

Q: How do I associate an Elastic IP with an instance in a Public Subnet using AWS CDK?

A: To associate an Elastic IP with an instance in a Public Subnet using AWS CDK, you can use the ec2.ElasticIp class and the apply_aspect method. Here is an example of how to associate an Elastic IP with an instance:

import aws_cdk as cdk
from aws_cdk import aws_ec2 as ec2
class MyStack(cdk.Stack):
def init(self, scope: cdk.Construct, id: str, **kwargs) -> None:
super().init(scope, id, **kwargs)
    # Create a VPC
    vpc = ec2.Vpc(self, &quot;VPC&quot;)

    # Create a Public Subnet
    public_subnet = ec2.Subnet(self, &quot;Public Subnet&quot;,
        vpc=vpc,
        cidr=&quot;10.0.1.0/24&quot;,
        map_public_ip_on_launch=True
    )

    # Create an instance in the Public Subnet
    instance = ec2.Instance(self, &quot;Instance&quot;,
        vpc=vpc,
        instance_type=&quot;t2.micro&quot;,
        machine_image=ec2.MachineImage.latest_amazon_linux(
            generation=ec2.AmazonLinuxGeneration.AMAZON_LINUX_2,
            edition=ec2.AmazonLinuxEdition.STANDARD,
            virtualization=ec2.AmazonLinuxVirt.HVM
        ),
        vpc_subnets=ec2.SubnetSelection(subnets=[public_subnet])
    )

    # Associate an Elastic IP with the instance
    elastic_ip = ec2.ElasticIp(self, &quot;Elastic IP&quot;,
        vpc=vpc
    )
    instance.apply_aspect(ec2.Aspect.ElasticIp(elastic_ip))

Q: What are some common issues that I may encounter when trying to get Elastic IPs associated with Public Subnets using AWS CDK?

A: Some common issues that you may encounter when trying to get Elastic IPs associated with Public Subnets using AWS CDK include:

The instance is not in the Public Subnet.
The Elastic IP is not associated with the instance.
The VPC does not have a Public Subnet.
The Public Subnet does not have a valid CIDR block.

Q: How do I troubleshoot issues with getting Elastic IPs associated with Public Subnets using AWS CDK?

A: To troubleshoot issues with getting Elastic IPs associated with Public Subnets using AWS CDK, you can use the following steps:

Check the instance's subnet to ensure that it is in the Public Subnet.
Check the Elastic IP's association to ensure that it is associated with the instance.
Check the VPC's subnets to ensure that it has a Public Subnet.
Check the Public Subnet's CIDR block to ensure that it is valid.

Conclusion

In this article, we answered some frequently asked questions (FAQs) related to getting Elastic IPs associated with Public Subnets using AWS CDK. We covered topics such as creating Public Subnets, associating Elastic IPs with instances, and troubleshooting common issues. By following the steps outlined in this article, you should be able to successfully get Elastic IPs associated with Public Subnets using AWS CDK.

Future Work

In the future, we can improve this article by adding more FAQs and troubleshooting steps. We can also add more examples of how to use AWS CDK to get Elastic IPs associated with Public Subnets.

References

AWS CDK Documentation: https://docs.aws.amazon.com/cdk/latest/guide/getting_started.html
AWS EC2 Documentation: https://docs.aws.amazon.com/ec2/latest/UserGuide/using-eni.html
AWS Elastic IP Documentation: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip.html