AWS Subnet Issues – Resolving Invalid Subnet Flag in VPC CIDR Block

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My VPC CIDR block is 10.0.0.0/16
As I understand it, this should support 256 (effectively 254) subnets with 65534 hosts per subnet.

When I tried to create one subnet 10.8.0.0/24 and associate it with the VPC, I received this error:

Error creating subnet: 
  InvalidSubnet.Range: The CIDR '10.8.0.0/24' is invalid.

An address like 10.8.0.1 belongs to the subnet 10.8.0.0 which looks to be inside 10.0.0.0/16. Perhaps I am misguided in my understanding. Please explain where I went wrong.

Best Answer

Nope, 10.8.0.0/24 doesn't fall into 10.0.0.0/16. AWS is correct, sorry :)

In your 10.0.0.0/16 you can create subnets between 10.0.0.0/24 to 10.0.255.0/24, e.g. 10.0.8.0/24 would work.

Let's say we've got CIDR address 10.20.30.40 - that can fall for example into these subnets:

  • 10.0.0.0/8 - /8 means only the first byte (10.) in the address is the network address.
  • 10.20.0.0/16 - /16 means the first two bytes (10.20.) are network.
  • 10.20.30.0/24 - /24 means the first three bytes (10.20.30.) are network.
  • 10.20.30.40/32 - /32 covers the whole address (10.20.30.40) and sometimes this notation is used to explicitly say it's a host address. AWS uses that a lot.

Hope that helps :)

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