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I'm new to AWS and I'm looking to create a subnet. Whenever I try the defaults subnets under I get either the error message "IPv4 block sizes must be between a /16 netmask and /28 netmask." or "CIDR Address is not within CIDR Address from VPC."enter image description here

I'm not sure how to proceed. Please can anyone assist. Thank you

  • As mentioned, a VPC will have a large address space that is divided up in to smaller, non-overlapping subnets, often associated with different availability zones. You need to obtain a basic understanding of subnetting. – Appleoddity Aug 02 '22 at 05:28

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The image attached has the answer. You've already defined a huge subnet 172.31.0.0/20 the second subnet you tried to create 172.31.0.0/16 falls within that. You probably got VPC and subnets confused. A VPC (virtual private cloud aka virtual network) has many subnets, which can't overlap.

I suggest you delete all your subnets and create some /24 subnets, rather than /20 subnets.

Tim
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