AWS Certified Solutions Architect - Professional 2020

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Networking Quiz — IPv4

For this question

Which of these CIDR blocks and/or IP addresses are invalid for a private VPC or subnet on AWS? (Choose 3)

A.  –> If I create a VPC with this CIDR block I could have the in an instance

B. –> This is clear, I could create a VPC with

C. –> The greatest CIDR block allowed is /16

D. with subnet mask –> I cannot specify this in a VPC or subnet, I can change instead for or

The answer marks the A as an invalid, but If I create a VPC with this CIDR block I could have the in an instance, Couldn’t I?

Patrick Smith

The IP address is physically part of the CIDR block however not every address in the CIDR block is usable. The first and last addresses are reserved for network address and broadcast in this case .255 is the boardcast IP. Also keep in mind AWS doesn’t allow broadcast traffic. So remember by default CIDR blocks have the number of possible addresses -2 BUT for the purposes of AWS keeps 3 reserved addresses too so for a total of 5 unsable IP’s in each CIDR block.

Muhammad Arif Wicaksana

but if the CIDR block is as he mentioned, is not a broadcast address; it is a usable IP address. It really depends on the CIDR size


Why would with subnet mask be invalid? Doesn’t that translate to CIDR block

1 Answers

Hi Jheison,

You are right that is a perfectly usable address if you create a VPC using then maybe a single subnet for to allocate all those addresses to that single subnet.  I’ve updated that question to remove that as an invalid answer.



Correct me if I am wrong but the wording " –> This is clear, I could create a VPC with" seems incorrect. is not a private range for a VPC or subnet. It is a valid IP which can be referenced inside a VPC or subnet. Is that the point?

Jheison Rodriguez

I think so, that’s referring to a valid IP range that you can set when define a VPC or subnet.

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