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.