VPC Peering: Subnet Considerations


VPC Fundamentals
What is a VPC?
VPC Security and Control
VPC Connectivity
VPC Sharing using the AWS Resource Access Manager
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3h 12m

In this section of the AWS Certified Advanced Networking - Specialty learning path, we introduce you to the various networking and VPC services currently available in AWS that are relevant to the ANS-C01 exam.

Learning Objectives

  • Identify and describe the various networking services available in AWS
  • Describe how to configure an Amazon Virtual Private Cloud (VPC)
  • Understand how to control network traffic via Security Groups and Network Access Control Lists (NACLs)
  • Describe options for VPC connectivity, subnets, and routing
  • Understand how to share VPC resources using the AWS Resource Access Manager (RAM)
  • Identify how to evaluate the configuration of VPC resources using the VPC Reachability Analyzer


The AWS Certified Advanced Networking - Specialty certification has been designed for anyone with experience designing, implementing, and operating complex AWS and hybrid networking architectures. Ideally, you’ll also have some exposure to the nuances of AWS networking, particularly regarding the integration of AWS services and AWS security best practices. Many exam questions will require advanced level knowledge of many AWS services, including AWS networking services. The AWS Cloud concepts introduced in this course will be explained and reinforced from the ground up.


Hello, and welcome to this lecture, where I shall be discussing subnets when used for VPC peering.

Let me start by explaining what VPC peering is at a high level. VPC peering allows you to connect two or more VPCs together, using IPV4 or IPV6, as if they were a part of the same network.

Once the peer connectivity is established, resources in one VPC can access resources in the other. The connectivity between the VPCs is implemented through the existing AWS network infrastructure, and so it is highly available with no bandwidth bottleneck. As peered connections operate as if they were part of the same network, there are restrictions when it comes to your CIDR block ranges that can be used.

If you have overlapping or duplicate CIDR ranges for your VPC, then you'll not be able to peer the VPCs together. So this is a design consideration if you plan on setting up multiple VPCs that you want to peer. Also from a design perspective, you are not able to daisy-chain VPCs together expecting them all to talk across one large network.

Each AWS VPC will only communicate with its peer. As an example, if you have a peering connection between VPC 1 and VPC 2, and another connection between VPC 2 and VPC 3 as shown, then VPC 1 and 2 could communicate with each other directly, as can VPC 2 and VPC 3, however, VPC 1 and VPC 3 could not. You can't route through one VPC to get to another.

To allow VPC 1 and VPC 3 to talk directly, you would have to implement a separate peering connection, between VPC 1 and VPC 3 as shown. Interestingly, the daisy-chain diagram allows the possibility of having two of the VPCs to have overlapping or even identical VPC CIDR blocks.

Let's say VPC 1 and VPC 3 have exactly the same CIDR block, which both connect to VPC 2. As the peer does not exist between VPC 1 and VPC 3, this is allowed without causing an issue. You may be wondering how this works from a routing perspective, if VPC 2 has two VPCs with the same CIDR block range, where does it know where to send traffic? I shall be covering VPC routing in an upcoming lecture, where this scenario will be discussed and explained.

Now, if we applied the same CIDR block settings in this example to the second scenario, where VPC 1 and VPC 3 were also peered, we would encounter configuration issues, and the peer connection would not be allowed due to overlapping CIDR blocks. We have now come to the end of this lecture covering VPC peering subnet considerations,

in the next lecture I shall be discussing flow logs, and how these can be used to monitor network traffic in and out of your subnets.


About the Author
Learning Paths

Stuart has been working within the IT industry for two decades covering a huge range of topic areas and technologies, from data center and network infrastructure design, to cloud architecture and implementation.

To date, Stuart has created 150+ courses relating to Cloud reaching over 180,000 students, mostly within the AWS category and with a heavy focus on security and compliance.

Stuart is a member of the AWS Community Builders Program for his contributions towards AWS.

He is AWS certified and accredited in addition to being a published author covering topics across the AWS landscape.

In January 2016 Stuart was awarded ‘Expert of the Year Award 2015’ from Experts Exchange for his knowledge share within cloud services to the community.

Stuart enjoys writing about cloud technologies and you will find many of his articles within our blog pages.