The answer is to create new public subnets with larger CIDR blocks, such as /23, and associate them with the same route table. This resolves the insufficient IP addresses for ALB subnets because each internet-facing Application Load Balancer requires at least eight free IP addresses per subnet to scale its elastic network interfaces, and the existing /24 subnets have only two free addresses left. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of ALB subnet IP requirements and the fact that private subnets cannot host internet-facing load balancers, a common trap where candidates mistakenly suggest using private subnets or shrinking existing ones. A key memory tip: ALBs need room to grow—always ensure at least eight free IPs per subnet, and when exhausted, add larger subnets rather than modifying existing ones.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company has a VPC with a CIDR of 10.0.0.0/16. It has two public subnets (10.0.1.0/24 and 10.0.2.0/24) and two private subnets (10.0.3.0/24 and 10.0.4.0/24). The company hosts a web application on EC2 instances in the private subnets behind an Application Load Balancer (ALB) in the public subnets. The ALB is internet-facing. The company wants to add a second ALB in the same VPC for a different application but using the same public subnets. The new ALB also needs to be internet-facing. However, when the company tries to create the new ALB, they receive an error: 'The subnet 'subnet-xxxxxxxx' does not have enough free IP addresses to satisfy the request.' The network engineer checks the subnets and finds that the public subnets have only 2 free IP addresses each. The private subnets have plenty of free IP addresses. The company wants to resolve this error without changing the architecture of the existing applications. What should the network engineer do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create new public subnets with larger CIDR blocks (e.g., /23) and associate them with the same route table.
Option B is correct. The public subnets are running out of IP addresses. Adding new public subnets with larger CIDR blocks (e.g., /23) will provide more IP addresses without affecting the existing subnets. Option A is incorrect because the private subnets cannot be used for internet-facing ALBs. Option C is incorrect because shrinking existing subnets would disrupt running resources. Option D is incorrect because launching a NATgateway does not add IP addresses to subnets.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Create new public subnets with larger CIDR blocks (e.g., /23) and associate them with the same route table.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create new public subnets with larger CIDR blocks (e.g., /23) and associate them with the same route table. — Option B is correct. The public subnets are running out of IP addresses. Adding new public subnets with larger CIDR blocks (e.g., /23) will provide more IP addresses without affecting the existing subnets. Option A is incorrect because the private subnets cannot be used for internet-facing ALBs. Option C is incorrect because shrinking existing subnets would disrupt running resources. Option D is incorrect because launching a NAT gateway does not add IP addresses to subnets.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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