AWS Certified Solutions Architect - Professional 2020

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How is the broadcast address?

In Quiz, it is said that these three

are reserved as it is the broadcast address.

I got last two, but i don’t see how the first one is the broadcast address

2 Answers

Hi there,
The wording for the explanation is a little confusing and the link provided doesn’t really address the question at hand.

I was able to find the following question in the quiz, which I believe is the one you’re referring to:

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

The problem with the answers are, only 2 of them are correct ( & As the explanation states, is a well known Google DNS server IP. The address is not reserved, but if the mask is 25, it would be. I think this is a typo in the quiz answer. I have notified ACG content team to review the question.


Thanks a lot!

Hi, yes it is a typo. "the first 3 are reserved as it is the broadcast address", actually means the first 3 address of each VPC or Subnet are reserved and so is the last address as that one is used for broadcasting and AWS does not allow broadcasting.

There 5 reserved IP on each CIDR:

.0 is the network address ( or the base address)

.1 is for the router (or base +1)

.2 is for the DNS (or base +2) – is thus invalid

.3 is reserved by AWS for future use

.255 is the broadcasting address. – is thus invalid

Furthermore the valid CIDR ranges are /16 to /28 – ( is thus invalid) 

That leaves as the only valid IP address. Even though this is a well known Google DNS server IP (a public IP address) , AWS allows, but does not recommend, the use of that address inside your VPC as VPC CIDR addresses are all private address. 

Ian Packer

Isn’t the convention 10.2.4.X/24 represents a subnet not a single address, therefore it’s valid?

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