IPv4 CIDR Block for Hosting 20 EC2 Instances
Question
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?
Answers
Explanations
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A. B. C. D.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-ipv4To determine the appropriate IPv4 CIDR block for a new subnet in AWS VPC that will host 20 EC2 instances, we need to first calculate the number of IP addresses required for the instances and other network resources.
Each EC2 instance needs at least one private IP address assigned to it. Additionally, we may require IP addresses for other network resources such as a NAT gateway, a VPN gateway, and DNS servers.
Assuming that we need 1 IP address for each instance and 4 IP addresses for other network resources, the total number of IP addresses required would be:
Total IP addresses = Number of instances + Number of network resources = 20 + 4 = 24
Next, we need to choose an IPv4 CIDR block that can accommodate at least 24 IP addresses. In AWS VPC, we can choose from the following predefined CIDR blocks:
- /16 (65,536 IP addresses)
- /20 (4,096 IP addresses)
- /24 (256 IP addresses)
- /28 (16 IP addresses)
- /29 (8 IP addresses)
- /30 (4 IP addresses)
Out of these, the smallest CIDR block that can accommodate at least 24 IP addresses is /27. This CIDR block provides 32 IP addresses, out of which 5 are reserved by AWS, leaving us with 27 usable IP addresses.
Therefore, the correct answer is option A: 151.0.0.0/27. This CIDR block can accommodate up to 32 IP addresses, out of which we can use 27 for our EC2 instances and other network resources.
