What Happens When Creating Subnets with Overlapping CIDR Blocks?
Question
A user has created a VPC with CIDR 20.0.0.0/16
The user has created one subnet with CIDR 20.0.0.0/16 in this VPC.
The user is trying to create another subnet with the same VPC for CIDR 20.0.0.0/24
What will happen in this scenario?
Answers
Explanations
Click on the arrows to vote for the correct answer
A. B. C. D.Answer - A.
Since the CIDR of the new subnet overlaps with that of the first subnet, an overlap error will be displayed.
See the snapshot below.
For more information on VPC subnets, please refer to the below link.
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html
The correct answer is A. It will throw a CIDR overlap error.
A Virtual Private Cloud (VPC) is a virtual network dedicated to the user's AWS account. When a user creates a VPC, they specify a range of IPv4 addresses for the VPC in the form of a Classless Inter-Domain Routing (CIDR) block. The CIDR block is a range of IP addresses in CIDR notation that is reserved for the VPC. For example, a CIDR block of 10.0.0.0/16 represents a range of IP addresses from 10.0.0.0 to 10.0.255.255.
When a user creates a subnet in a VPC, they specify a range of IP addresses from the VPC CIDR block to allocate to the subnet. A subnet is a range of IP addresses in the VPC CIDR block that can be used to launch Amazon Elastic Compute Cloud (EC2) instances, Amazon Relational Database Service (RDS) instances, and other resources.
In this scenario, the user has created a VPC with CIDR 20.0.0.0/16 and one subnet with CIDR 20.0.0.0/16. The user is trying to create another subnet with CIDR 20.0.0.0/24 in the same VPC. However, this is not possible as the CIDR blocks of the subnets are overlapping.
CIDR block 20.0.0.0/16 represents a range of IP addresses from 20.0.0.0 to 20.0.255.255. CIDR block 20.0.0.0/24 represents a range of IP addresses from 20.0.0.0 to 20.0.0.255. As we can see, the CIDR block 20.0.0.0/24 is a subset of CIDR block 20.0.0.0/16. This means that the second subnet's IP address range falls within the IP address range of the first subnet, causing an overlap.
Therefore, when the user tries to create the second subnet with CIDR 20.0.0.0/24, it will throw a CIDR overlap error, and the user will not be able to create the subnet with the same IP address range as the existing subnet in the VPC.
Option A is the correct answer as the CIDR overlap error will prevent the creation of a subnet with an overlapping IP address range. Option B is incorrect because it is possible to create a subnet with a different CIDR block within the VPC. Option C and D are incorrect because the second subnet cannot be created due to the CIDR overlap error.
