CIDR Address overlaps with existing Subnet CIDR

Question

I am trying to create an application load balancer. To do this, I need two subnets.

The current subnet CIDR block is 172.31.0.0/16. Every time I try to create a new CIDR block for the 2nd subnet, I keep getting this error:

CIDR Address overlaps with existing Subnet CIDR

I am not a big networking guy. So I am struggling with this.

I've tried googling but I'm struggling to comprehend what to actually input.

Answer 1

A /16 CIDR is an extremely large range to allocate to a subnet.

Start by taking a look at the CIDR allocated to the whole VPC. It is likely to be something like 172.31.0.0/16. A /16 range has 65,536 IP addresses.

Then, take a look at the subnets. They are likely to be something like 172.31.0.0/24 and 172.31.1.0/24. A /24 range has 256 IP addresses.

It is safer to always use a CIDR calculator such as: https://cidr.xyz/

For more information about private IP address ranges, see: Private network - Wikipedia