The CIDR is invalid AWS Cloud Formation

Question

I am using cloud formation to create instances, I am using following CIDR configurations for VPC and subnets.

VPC CIDR: "10.15.0.0/16" 
Subnet A CIDR: "10.15.10.0/24"
Subnet B CIDR: "10.15.20.0/24"
PrivateSubnet CIDR A: "10.15.90.0/21"
PrivateSubnet CIDR B: "10.15.110.0/21"

I am getting following error. Please let me know what I am doing wrong here ?

The CIDR '10.15.20.0/24' is invalid. (Service: AmazonEC2; Status Code: 400; Error Code: InvalidSubnet.Range; )

Answer 1

10.15.20.0/24 is fine.

However, 10.15.90.0/21 and 10.15.110.0/21 are invalid.

• 10.15.90.0/21 falls within 10.15.88.0 to 10.15.95.255, so it should really be 10.15.88.0/21
• 10.15.110.0/21 falls within 10.15.104.0 to 10.15.111.255, so it should really be 10.15.104.0/21

CIDR notation hurts the head, so you always use a CIDR Calculator.

If you don't follow the above concepts, read Classless Inter-Domain Routing - Wikipedia.