Potential Impact of IP Address Conflict: Same CIDR Block for Multiple VPCs

Question

Creating a VPC with a specific CIDR block, such as 10.0.0.0/16, provides a unique IP address range within your network. However, there is a possibility that someone else in the world might also create their VPC with the same CIDR block. What happens if someone creates their VPC with same CIDR block as I do, will I face any issues ?

Answer

No, there is no requirement to keep the IP ranges in VPC world-wide unique accross AWS customers. You can freely chose the CIDR blocks of your VPCs.

The technology behind that is explained in this re:Invent session, if you are interested: https://www.youtube.com/watch?v=3qln2u1Vr2E&t=4s&pp=ygUjYW5vdGhlciBkYXkgYW5vdGhlciBtaWxsaW9uIHBhY2tldHM%3D

**However**: Unique CIDR block get relevant when you want to create IP level connections between the VPCs and other VPCs or on-premises network. In this case duplicate IP ranges will lead to routing issues. So please make sure to have on overarching IP address plan that breaks down to your VPCs and on-premises networks. Services like AWS IPAM can help you with automating the allocation of CIDR blocks.

Potential Impact of IP Address Conflict: Same CIDR Block for Multiple VPCs

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