Understanding CIDR Block : A Case Study .

Understanding CIDR Block : A Case Study .

Overlapping in CIDR in AWS VPC


When setting up a Virtual Private Cloud (VPC) in Amazon Web Services (AWS), one of the critical considerations is the allocation of IP addresses. Proper IP address management within a VPC is crucial to ensure that your resources are isolated and secure. A common issue that AWS users may encounter is CIDR block overlapping, which can lead to network configuration problems and conflicts.

In this blog post, I'll walk you through a real-world case study where a VPC's CIDR block overlapped with a subnet's CIDR block, causing issues when trying to create additional subnets. We'll explore the problem, its implications, and how to resolve it.

The Scenario

Let's dive into the scenario. Imagine you are a cloud engineer tasked with creating a VPC in AWS. You decide to define your VPC with a CIDR block of "" This CIDR block specifies a range of IP addresses from "" to "," which provides 256 available IP addresses for your VPC's resources.

The First Subnet: Subnet1

As you proceed with creating resources within your VPC, you decide to create a subnet named "subnet1." However, in a moment of oversight, you assign the same CIDR block "" to "subnet1" as you did for your VPC.

While creating "subnet1," AWS allows you to proceed with this configuration since it is only checking the uniqueness of CIDR blocks within the context of the specific subnet. This, however, is where the problem begins.

The Issue: CIDR Block Overlapping

CIDR block overlapping is a situation where two or more subnets within the same VPC have the same CIDR block. In this case, the VPC's CIDR block and "subnet1's" CIDR block are identical, and they are overlapping. When you attempt to create another subnet, say "subnet2," you will encounter issues.

The Implications

The implications of CIDR block overlapping within a VPC are significant:

1. Conflict and Error: AWS will not allow you to create a new subnet with the same CIDR block as an existing subnet within the same VPC. As a result, you will receive an error message.

2. IP Address Confusion: Overlapping CIDR blocks can lead to IP address conflicts and make it challenging to manage and route network traffic.

3. Routing and Security Issues: Subnets are often used to define routing and security rules. Overlapping CIDR blocks can disrupt these configurations, leading to unintended and unpredictable network behaviors.

My Personal Experience and the Resolution

In my journey of setting up a VPC in AWS, I encountered a challenge that I'd like to share with you. I defined my VPC with a CIDR block of "," an address range from "" to "" However, when I created my first subnet, "subnet1," I mistakenly assigned it the same CIDR block, "," as my VPC.We should never give the subnet same CIDR as the VPC and give the subnet a CIDR within the VPC range.Here's When the problem started.

At first, AWS allowed me to create "subnet1" with this configuration since it is only checking for uniqueness within the context of the specific subnet. However, this mistake became apparent when I attempted to create another subnet, "subnet2."

To fix the CIDR block overlapping problem, I simply had to choose a different CIDR block for "subnet2" that didn't conflict with the existing "" CIDR block of "subnet1 and also change the CIDR to a bigger range. In this case, I opted for for the VPC, for subnet1 and for subnet 2. This choice ensured that "subnet2" had its unique IP address range while still residing within the VPC's overall address space.

This experience taught me the importance of careful planning and documentation when it comes to AWS VPC configuration. To prevent CIDR block overlapping, it's crucial not to use the same CIDR block for a subnet as the one assigned to the VPC. Following these best practices can save us valuable time and prevent configuration hiccups in the long run.

Best Practices

To avoid CIDR block overlapping and similar issues, consider the following best practices when designing your AWS VPC:

1. Plan CIDR Blocks Carefully: Choose CIDR blocks that do not overlap within the same VPC.

2. Document Your IP Addressing: Keep records of your VPC's CIDR blocks and the CIDR blocks assigned to each subnet.

3. Regularly Review and Audit: Periodically review your VPC's configuration to ensure there are no conflicts or overlapping CIDR blocks.

4. Automate Where Possible: Use infrastructure as code (IaC) and automation tools to manage your VPC configurations. These tools can help ensure consistency and prevent human errors.

In conclusion, my experience with CIDR block overlapping emphasized the need for thoughtful planning and adherence to AWS best practices. It's always a good idea to learn from the challenges we face, ensuring that our VPC environments are well-organized, conflict-free, and easier to manage.

The next article is Regarding of 'How to Create a VPC in AWS'. Follow and Subscribe to the Newsletter please.