- A
Cross-zone load balancing is disabled on the NLB.
Why wrong: Disabled cross-zone can cause uneven distribution but not timeouts if instances in all AZs.
- B
The EC2 instances' security group allows traffic from the NLB's security group, but the NLB preserves client IP, so the security group must allow client IPs directly.
NLB preserves source IP; security group references to NLB's security group only work for traffic that originates from NLB's private IPs, but NLB sends client IPs.
- C
The NLB's security group is not allowing traffic from the EC2 instances.
Why wrong: NLB does not have a security group; it uses security groups for targets.
- D
The network ACLs in the private subnets are blocking return traffic.
Why wrong: Network ACLs are stateless and need outbound ephemeral ports open; this could be a cause but less likely than security group.
Quick Answer
The answer is that the EC2 instances' security group must allow traffic directly from the client IPs, not just from the NLB’s security group, because the NLB preserves the source IP by default. When troubleshooting NLB connection timeouts due to security group source IP preservation, the key technical concept is that an internet-facing Network Load Balancer forwards packets with the original client IP address, not the NLB’s own private IP. This means the security group rule referencing the NLB’s security group is effectively ignored, as the traffic arrives from the client’s IP instead. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of NLB behavior versus Application Load Balancer behavior—a common trap is assuming all load balancers rewrite the source IP. Remember the mnemonic: “NLB keeps the client, ALB keeps the node”—NLB preserves the client IP, while ALB uses its own private IP. For a quick memory tip: if you see intermittent timeouts with an NLB, always check whether the security group allows the actual client CIDR ranges.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 public and private subnets in three Availability Zones. An internet-facing Network Load Balancer (NLB) is deployed in the public subnets, and a fleet of EC2 instances is in the private subnets. The application logs show intermittent connection timeouts. The security group for the EC2 instances allows traffic from the NLB's security group. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The EC2 instances' security group allows traffic from the NLB's security group, but the NLB preserves client IP, so the security group must allow client IPs directly.
Option C is correct because NLB's source IP preservation by default sends traffic with client IP, not NLB's private IP, so security group must allow client IPs. Option A is wrong because subnet ACLs are stateless and need ephemeral ports open. Option B is wrong because NLB does not have a security group. Option D is wrong because cross-zone load balancing is enabled by default for NLB.
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.
- ✗
Cross-zone load balancing is disabled on the NLB.
Why it's wrong here
Disabled cross-zone can cause uneven distribution but not timeouts if instances in all AZs.
- ✓
The EC2 instances' security group allows traffic from the NLB's security group, but the NLB preserves client IP, so the security group must allow client IPs directly.
Why this is correct
NLB preserves source IP; security group references to NLB's security group only work for traffic that originates from NLB's private IPs, but NLB sends client IPs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The NLB's security group is not allowing traffic from the EC2 instances.
Why it's wrong here
NLB does not have a security group; it uses security groups for targets.
- ✗
The network ACLs in the private subnets are blocking return traffic.
Why it's wrong here
Network ACLs are stateless and need outbound ephemeral ports open; this could be a cause but less likely than security group.
Common exam traps
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: The EC2 instances' security group allows traffic from the NLB's security group, but the NLB preserves client IP, so the security group must allow client IPs directly. — Option C is correct because NLB's source IP preservation by default sends traffic with client IP, not NLB's private IP, so security group must allow client IPs. Option A is wrong because subnet ACLs are stateless and need ephemeral ports open. Option B is wrong because NLB does not have a security group. Option D is wrong because cross-zone load balancing is enabled by default for NLB.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
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