AWS Certified Solutions Architect - Professional 2020

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IPV4 Conventions

The question; "Which of these CIDR blocks and/or IP addresses are invalid for a private VPC or subnet on AWS? (Choose 3)", in the Networking Quiz in the AWS SA Pro 2019 course it states: is not a valid address on the basis that "255" is the broadcast. But … isn’t this nomenclature merely specifying the subnet and the number 255 (or anything) on the end is irrelevant?

Similarly it states is valid! Well it might be a valid IPV4 address but it’s not a valid VPC range is it?

1 Answers

Hi Ian,

I can see the confusion…I was trying to specify a CIDR block in some answers and a subnet mask in others.   I updated the question to be less confusing.   

However, is still an invalid CIDR for a VPC or Subnet…seems there is validation that ensures the last position of the CIDR is valid for the subnet.  Try to enter that CIDR in the console for a subnet…


Ian Packer

Using the console as the source of truth; doesn’t seem to get accepted.

Scott Pletcher netmask is a valid IP address for a machine in a VPC. To use it as a CIDR, you’d need to enter it as for a netmask or for a netmask for example.

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