VPC operation and isolation: physical and software: How it works ?

Question

Hi,  
I would like to know how VPC works in deeper.  
According to the doc, each VPC is isolated from others:   https://docs.aws.amazon.com/vpc/latest/userguide/what-is-amazon-vpc.html

But for me, it's not enough for understanding how it works.

For example:  
We have Client A and Client B.  
Both would like to create 2 VPC with sane CIDR Range: 10.1.0.0/16 in the same region and same availability zone. So here my questions:

- Can both have same CIDR Range in the same region and same AZ ?
- Can client A or B has the same CIDR range for its 2nd VPC ?
- Is a VPC like a virtual router or virtual switch ?
- VPC operates on software level if I'm not wrong, how the software level handles same CIDR range ? How isolation operates ?

Can anyone help me please ?

Thank you !

[Edit] I updated my english (sorry if it's not really good) and the render (markdown)

Brettski-AWS · Accepted Answer

VPCs are logical network constructs. You can have two VPCs with the same IP range but that makes it quite difficult for those VPCs to communicate. More information: https://aws.amazon.com/blogs/networking-and-content-delivery/connecting-networks-with-overlapping-ip-ranges/

You might find this video interesting as it goes into a lot of detail about how VPC (and other AWS networking components) work: https://youtu.be/8gc2DgBqo9U - under the hood, VPC is performing routing and switching tasks for you.

As far as hypervisors go, all modern AWS instances use Nitro which is a hardware-based hypervisor: https://aws.amazon.com/ec2/nitro/

re:Post Agent · Answer

Amazon Virtual Private Cloud (VPC) is a sophisticated service that provides logical isolation for AWS resources. Here's a deeper explanation of how VPCs work and how they handle isolation:

1. CIDR Range Overlap:
   - Yes, Client A and Client B can have VPCs with the same CIDR range (10.1.0.0/16) in the same region and availability zone. VPCs are logically isolated from each other, even if they have overlapping IP address ranges.
   - A client can also create multiple VPCs with the same CIDR range within their account.

2. VPC Nature:
   - A VPC is more like a virtual network than just a router or switch. It encompasses multiple networking components, including subnets, route tables, network gateways, and security controls.

3. Software-Level Isolation:
   - VPC operates at the software level, utilizing AWS's networking infrastructure to create logical separation.
   - The isolation is achieved through advanced network virtualization techniques. Each VPC is assigned a unique identifier, and all traffic within the VPC is tagged with this identifier.
   - When packets are processed, AWS's networking layer uses these identifiers to ensure that traffic from one VPC never crosses into another VPC, even if they have overlapping IP ranges.

4. Security and Isolation:
   - VPCs provide robust logical isolation. Every packet flow is individually authorized, making unauthorized information transfer between VPCs highly improbable.
   - Multiple layers of security are employed, including security groups and network access control lists (ACLs), to protect resources within each subnet.

5. Connectivity and Encryption:
   - All traffic within a VPC and inter-region peering is encrypted.
   - When traffic must travel outside of AWS's control, physical network encryption is used to maintain security.

In essence, VPCs create secure, isolated network environments within the AWS cloud. The software-defined networking capabilities allow for flexible IP addressing and network configurations while maintaining strict isolation between different customers' resources, even when they share physical infrastructure.  
**Sources**  
[VPC and Accompanying Features - Logical Separation on AWS](https://docs.aws.amazon.com/whitepapers/latest/logical-separation/vpc-and-accompanying-features.html)  
[How Amazon VPC works - Amazon Virtual Private Cloud](https://docs.aws.amazon.com/vpc/latest/userguide/how-it-works.html)  
[Community | AWS Security Essentials](https://community.aws/concepts/security-essentials)

VPC operation and isolation: physical and software: How it works ?

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