AWS Certified Solutions Architect - Professional 2020

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Subnet Mask IPs

I have a VPC with CIDR For a /28 subnet, why does a CIDR need to have to be a multiple of 16. Why cant I have or if I want to have an IP starting with say Why does it have to be in counts of 16. like or ​​

1 Answers

The answer lies in how the IP addresses are represented in binary form. To learn more I really suggest starting by reading the following Wiki article: Classless Inter-Domain Routing and then playing around with a tool like that offered at


Thanks for a quick response. I had gone through the links. can also be represented in binary as well. So why cant I start a subnet with if I just need 16 IPs.

Tom Kringstad

Keep in mind that IP addressing is zero-based. i.e. The following are both valid CIDR blocks with 16 IP addresses., In your example, would be the first IP address in the address block.

Tom Kringstad

Go in to the VPC console and create a new VPC with CIDR block. Then try creating a subnet with address block. You’ll get an error message. Change it to a address block and you’ll have success.

Tom Kringstad

Play a bit more with the tool at Enter, then change to,, etc. You’ll see the network mask stays the same, and that the network address for is

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