Long time ago, from 04/Dec/2013 AWS release Virtual Private Cloud (VPC) and make VPC as default for new AWS account. VPC enables you to define a virtual network in your own logically isolated area within the AWS cloud.
VPC is the big revolution of AWS Cloud but when user had created VPC, they are no longer to modify CIDR size of VPC. There are only 1 way when you want to change CIDR size of VPC that is you must delete it and create another one with expected CIDR size. But, what will be happened when your system is running with many other services. It’s not easy to delete.
So, almost case, you choose biggest CIDR block that is supported:
10.0.0.0–10.255.255.255(10/8 prefix)172.16.0.0–172.31.255.255(172.16/12 prefix)192.168.0.0–192.168.255.255(192.168/16 prefix)
When you create VPC with big CIDR size, there are some pros and cons. I will list as below:
- Pros:
- Reserved IP block for future using
- Long IP range for Auto Scaling Group
- Cons:
- As big as you used now, as less as CIDR Blocks leave to use in the future: This means in the big system, you may connect all VPCs together (VPC Peering, VPN Connection,…) and CIDR cannot overlap each others. So your system’s network will be limited in the future.
As new update on 20/Sep/2017, now user still cannot resize of VPV CIDR Block but user can expand it. With this update, that just an additional CIDR Blocks for your existing VPC, not resize/modify.
Limitation: 5 IPv4 CIDR blocks per VPC. This limit is made up of your primary CIDR block plus 4 secondary CIDR blocks but you can create request to AWS to increase this limit.
How to modify: You can use CLI or Console to modify
How it work: After add more new CIDR Blocks for existing VPC, you can see them in main route table.
Reference: