Creating Subnet IP address : IPv4 block sizes must be between a /16 netmask and /28 netmask

Question

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

score 1 · Answer 1 · answered Aug 01 '22 at 03:42

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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.

answered Aug 01 '22 at 03:42

Tim

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When I do that I still get the error message – Christian Fuh Aug 01 '22 at 21:28
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Please edit your question to show screenshots of your VPC (including CIDR) and your subnets (including CIDR) then comment below so I get a notification. – Tim Aug 02 '22 at 00:24

Creating Subnet IP address : IPv4 block sizes must be between a /16 netmask and /28 netmask

1 Answers1