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If I set up an AWS VPC with CIDR of 10.0.0.0/20 then the range is 10.0.0.0 to 10.0.15.255
I make a subnet now. Why does 10.0.15.15/28 give an error. Isn't it's range 10.0.15.15 to 10.0.15.30 which is within the VPC?

Akram ElectroTech
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  • /28 is 16 addresses. 0-15, 16-31, etc. (0 and 15 -- all-zeros and all-ones addresses -- are reserved for network and broadcast) – Ricky Jan 08 '21 at 02:20

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Isn't it's range 10.0.15.15 to 10.0.15.30 which is within the VPC?

Yes, but it's not a valid /28.

10.0.15.0/28 or 10.0.15.16/28 are valid ranges.

Ron Trunk
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    For more details on CIDR subnetting, check out this primer over at Server Fault: https://serverfault.com/questions/12854/cidr-for-dummies – RobinG Jan 07 '21 at 19:06
  • Why isn't it a valid /28 and why should it be a multiple of 16 ? – Akram ElectroTech Jan 07 '21 at 19:08
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    A /28 subnet requires the lowest four bits - the host bits - to be all zero. – Zac67 Jan 07 '21 at 19:16
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    @AkramElectroTech, if you read and understand [this two-part answer](https://networkengineering.stackexchange.com/a/53994/8499), you will see where you have gone wrong. – Ron Maupin Jan 07 '21 at 21:55