In your AWS VPC, you need to add a new subnet that will allow you to host a total of 20 EC2 instances. Which IPv4 CIDR block would you use to achieve the same?

Answer options:

A.151.0.0.0/27
B.151.0.0.0/28
C.151.0.0.0/29
D.151.0.0.0/30

Correct Answer: A
AWS reserves 5 IP addresses.
The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use, and cannot be assigned to an instance. For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved:
10.0.0.0: Network address.
10.0.0.1: Reserved by AWS for the VPC router.
10.0.0.2: Reserved by AWS. The IP address of the DNS server is the base of the VPC network range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. We also reserve the base of each subnet range plus two for all CIDR blocks in the VPC. For more information, see the Amazon DNS server.
10.0.0.3: Reserved by AWS for future use.
10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address.
The formula to calculate the number of assignable IP addresses to CIDR networks is similar to classful networking. Subtract the number of network bits from 32. For example, a /24 network has 2^(32-24) - 2 addresses available for host assignment.
A.Prefix Length is ‘27’
Therefore 32-27 = 5 and 2 ^ 5 (i.e 2 * 2 * 2 * 2 * 2) – 5 = 27
B.Prefix Length is ‘28’
Therefore 32-28 = 4 and 2 ^ 4 (i.e 2 * 2 * 2 * 2) - 5= 11
C.Prefix Length is ‘29’
Therefore 32-29 = 3 and 2 ^ 3 (i.e 2 * 2 * 2) - 5 = 3
D.Prefix Length is ‘30’
Therefore 32-30 = 2 and 2 ^ 2 (i.e 2 * 2) - 5 = -1
For option ‘A’, we get ‘27’ IP addresses (or indirectly the number of instances to be provisioned) as shown above.
Since the user has to provision ‘20’ EC2 instances, we need to go with option ‘A’, which is the only correct IPv4 CIDR block.
Option B is incorrect because we get only ‘11’ IP addresses (or indirectly the number of instances to be provisioned ).
Option C is incorrect because we get only ‘3’ IP addresses.
Option D is incorrect because we get only ‘-1’ IP address.
For more information, please refer to the below URL-
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html#vpc-sizing-ipv4