How overlapping AWS VPC CIDRs are managed in AWS?

Question

I am curious about how AWS manages VPC IP address range.

My question is how AWS manages VPC's with same CIDR range in multiple AWS accounts.

Lets say I have:

• 2 AWS accounts (Account-A & Account-B)
• In Account-A, If I create a VPC with CIDR as 10.0.0.0/26 it will obviously allow me to create the VPC.
• In Account-B in same region if I create another VPC with same CIDR as Account-A i.e 10.0.0.0/26 then this also will be created in Account-B.

How come two VPCs will have same CIDR in different AWS accounts -- Shouldn't they be globally unique?

I want to know how this is working in AWS networking.

Please enlighten me on this. Also, please let me know If I am thinking in wrong direction as well.

Thanks in Advance.

Answer 1

Amazon VPCs are software-defined networks, or virtual networks.

The CIDRs are not actually used on the networking equipment. Rather, the traffic is encapsulated within other packets and sent between virtual resources attached to the virtual network. It is designed to work like conventional networks, but is implemented differently.

AWS users do not need to 'know' how it is implemented -- they can simply define networks and trust that the traffic will work within each virtual network. This is similar to virtual networks used on VMware virtual networks.