The answer is that the security group for the EC2 instances does not allow traffic from the NLB. This is because a Network Load Balancer operates at layer 4 and does not have a security group itself; instead, it sends traffic to targets using its own private IP addresses from within the VPC subnet. If the target instances’ security group lacks an inbound rule permitting traffic from the NLB’s subnet CIDR or the NLB’s private IPs, the TCP handshake on port 443 will be silently dropped, causing client timeouts even when health checks pass. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that NLB traffic bypasses security group association on the load balancer itself, making target-side rules the critical failure point—a common trap is assuming health check success guarantees data-plane connectivity. Remember the mnemonic: “NLB has no SG; your targets must see its IP.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 deployed a Network Load Balancer (NLB) in front of a fleet of EC2 instances in a VPC. The NLB is configured with a TCP listener on port 443. Clients are experiencing timeouts. The target group health checks are passing. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the EC2 instances does not allow traffic from the NLB.
Option C is correct because security groups must allow traffic from the NLB's private IP addresses. Option A is incorrect because cross-zone load balancing affects distribution, not timeouts. Option B is incorrect because health checks passing means targets are healthy. Option D is incorrect because deletion protection prevents accidental deletion, not timeouts.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Why it's wrong here
Cross-zone balancing affects traffic distribution, not connectivity.
✗
Deletion protection is enabled on the NLB.
Why it's wrong here
Deletion protection does not affect traffic.
✓
The security group for the EC2 instances does not allow traffic from the NLB.
Why this is correct
NLB uses its own IP addresses; security group must allow them.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The target group is using an incorrect health check path.
Why it's wrong here
Health checks are passing, so the path is correct.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
Network Management and Operations — This question tests Network Management and Operations — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The security group for the EC2 instances does not allow traffic from the NLB. — Option C is correct because security groups must allow traffic from the NLB's private IP addresses. Option A is incorrect because cross-zone load balancing affects distribution, not timeouts. Option B is incorrect because health checks passing means targets are healthy. Option D is incorrect because deletion protection prevents accidental deletion, not timeouts.
What should I do if I get this ANS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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