Splitting a VPC into Subnets for IP Address Allocation

Configuring IP Address Allocation for Split VPC Subnets

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Question

There is a requirement to split a VPC with a CIDR block of 10.0.0.0/24 into two subnets, each of which should give 128 IP addresses (including the 5 fixed IPs reserved by AWS)

Can this be done and if so, how will the allocation of the IP addresses be configured? Choose the correct answer from the below options.

Answers

Explanations

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A. B. C. D.

Answer - C.

This is clearly given in the AWS documentation.

" For example, if you create a VPC with CIDR block 10.0.0.0/24, it supports 256 IP addresses.

You can break this CIDR block into two subnets, each supporting 128 IP addresses.

One subnet uses CIDR block 10.0.0.0/25 (for addresses 10.0.0.0 - 10.0.0.127) and the other uses CIDR block 10.0.0.128/25 (for addresses 10.0.0.128 - 10.0.0.255)".

For more information on VPC and subnets, please see the below link-

http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html

To get the IP addresses and subnets, please see the below link-

http://www.subnet-calculator.com/cidr.php
cidr / vism upernet calculator Subnets CIDR Wildcard Feedback [© ads by Google [A 792.168 1.1 > ¢ ‘AIlIP Network ) (“Bandwidth Control) Juniper Segment Routing - Simplify Network Management © CIDR Calculator CIDR Calculator sees) The CIDR Calculator enables CIDR network 10.0.0.0 calculations using IP address, subnet mask, mask bits, aS Wildcard Mask maximum required IP addresses and maximum Mas required subnets. 0.0.0.127 Results of the CIDR calculation provide the wildcard mask, for use with ACL (Access Control Lists), CIDR network address (CIDR route), network address in CIDR notation and the CIDR address range for the resulting CIDR network. CIDR Address Range For classful subnets, use the IP Subnet Calculator. For 10.0.0.0- 10.0.0.127 a smart-phone optimized CIDR / VLSM Calculator, visit IP Calc. For ACL (Access Control List) wildcard mask calculations, use the ACL Wildcard Mask Calculator. Instant Grammar Checker @ grammarly CIDR - Classless Inter Domain Routing CIDR - Classless Inter Domain Routing - was adopted to help ease the load imposed on internet and large network backbone routers by the increasing size of routing tables. ‘OX Juniper Segment Routing Large routing tables have several adverse effects: + Routers require more memory in order to store and | > FSS fon are apm | 9/28/2019 SeOeeERHSa*RZutew

To split a VPC with a CIDR block of 10.0.0.0/24 into two subnets, each of which should give 128 IP addresses (including the 5 fixed IPs reserved by AWS), we need to divide the /24 network into two smaller subnets of /25.

The /25 network provides 128 IP addresses, out of which 5 IP addresses are reserved by AWS for various purposes such as network address, broadcast address, etc. This means that each of the two subnets would have 123 usable IP addresses.

Option A suggests that we use CIDR block 10.0.0.0/127 (for addresses 10.0.0.0 - 10.0.0.127) and the other will use CIDR block 10.0.0.128/255 (for addresses 10.0.0.128 - 10.0.0.255). This option is incorrect because the subnet mask /127 only allows for two IP addresses, which is not sufficient to provide 123 usable IP addresses required by each subnet.

Option B suggests that we use CIDR block 10.0.0.0/25 (for addresses 10.0.0.0 - 10.0.0.127) and the other will use CIDR block 10.0.1.0/25 (for addresses 10.0.1.0 - 10.0.1.127). This option is incorrect because the two subnets are not contiguous, and it is not possible to have a contiguous CIDR block that spans two non-contiguous /25 subnets.

Option C suggests that we use CIDR block 10.0.0.0/25 (for addresses 10.0.0.0 - 10.0.0.127) and the other will use CIDR block 10.0.0.128/25 (for addresses 10.0.0.128 - 10.0.0.255). This option is correct because it provides two contiguous subnets of /25 each, with 123 usable IP addresses in each subnet.

Therefore, the correct answer is option C.

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