I need to understand how IPS are manage in entire AWS

Question

In a local network, almost all IPs are available, and each device receives a private IP managed by the (ISP) box. Only the box has a private IP AND a public IP to transfer the request to the Internet and receive the response, before transferring the response to the client device's private IP.
This is a simple implementation of the network; AWS is a little more complex.

When I create my account, a vpc is created for my account, I understand that a vpc is linked to a region and can depend on one or more ZAs.
But the ip of my vpc is 172.31.0.0/16, I can use from 172.31.0.0 to 172.31.255.255 for my resources. This means that aws, in my region, could assign the other IPs ( from 0.0.0.0 to 172.30.0.0 and from 172.32.0.0 to 255.255.0.0) to the other accounts created in my region, so 255*255 -1 (my account) =65024 other accounts in my region. This seems to be very small, so I guess I didn't understand how the ip from the vpc works. I also don't understand why I can create 2 vpc with the same cidr.

I can't find anything on the internet that really explains how ip works on aws.

Accepted Answer

I hope below explanation will give you an idea, how IPs are managed in AWS.

We create VPC in AWS with Private CIDR range.

Private IP can only allow certain values:
• 10.0.0.0 – 10.255.255.255 (10.0.0.0/8) 
• 172.16.0.0 – 172.31.255.255 (172.16.0.0/12) <= default VPC included in this range. Most of the time default AWS VPC gets created in this range.
• 192.168.0.0 – 192.168.255.255 (192.168.0.0/16)

All the rest of the IP on the internet are public IP.

Yes other accounts can create VPC with same CIDR range but that is private to their network. Well you cannot join those two VPCs/network/accounts because those are having overlapping CIDR and obviously creates conflicts.

AWS assigns private ip to resources from the CIDR range which is used to create VPC. You can choose specific private IP from CIDR range if you want.

And CIDR range 172.31.0.0/16, is quite big, it provides 65534 IP addresses.

Best Regards,
Vikas

Answer

Here you can find information about how IPs works in AWS: [IP addressing documentation](https://docs.aws.amazon.com/vpc/latest/userguide/vpc-ip-addressing.html).

Is important to differentiate between the [Private IPs ](https://docs.aws.amazon.com/vpc/latest/userguide/vpc-ip-addressing.html#vpc-private-ipv4-addresses) and the [Public IPs](https://docs.aws.amazon.com/vpc/latest/userguide/vpc-ip-addressing.html#vpc-public-ipv4-addresses):

* Private IPs: **are not reachable over the internet**, and can be used for communication between the instances in your VPC. This are the ones that appears in your VPC, as these are just for internal use there is no overlapping issues with another accounts private IPs.
* Public IPs: These are the ones that give you access to the internet. So these are the ones that have to be unique.

Usually most of the resources in your VPC will have just private IPs and you will just a few Public IPs to provide access to the elements in your VPC to the internet.

I need to understand how IPS are manage in entire AWS

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