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:
10.2.4.255/24 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 188.8.131.52/255.255.255.0 is valid! Well it might be a valid IPV4 address but it’s not a valid VPC range is it?
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, 10.2.4.255/24 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…