The answer is that the network ACL outbound rule is blocking return traffic, even though the inbound rule allows HTTP. This occurs because network ACLs are stateless, meaning they evaluate each packet independently without tracking connection state. When a web server responds to an incoming HTTP request, the return traffic uses a random ephemeral port (typically 1024-65535) as the destination, and if the outbound rule does not explicitly allow this ephemeral port range, the response packet is dropped. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of the fundamental stateless versus stateful firewall behavior—a common trap is assuming that allowing inbound traffic automatically permits the reply, which is true for security groups but not for NACLs. A reliable memory tip is to remember that NACLs are “stateless and need both directions,” while security groups are “stateful and handle the return flow.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 public subnet hosting a web server. The security group for the web server allows inbound HTTP (port 80) from 0.0.0.0/0. The network ACL for the public subnet allows inbound HTTP from 0.0.0.0/0. Users report that they cannot access the website. The engineer verifies that the web server is running and has a public IP. What is the most likely issue?
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 network ACL outbound rule is blocking return traffic.
Even if inbound rules allow traffic, if the network ACL's outbound rule (stateless) does not allow return traffic (ephemeral ports), the connection will fail. Security groups are stateful and allow return traffic automatically.
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.
✗
The web server is listening on a different port.
Why it's wrong here
If the server were on a different port, the security group would block it.
✓
The network ACL outbound rule is blocking return traffic.
Why this is correct
Network ACLs are stateless, so return traffic on ephemeral ports must be explicitly allowed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
If the IGW were missing, no traffic would reach the server at all.
✗
The security group outbound rule is blocking return traffic.
Why it's wrong here
Security groups are stateful; outbound rules do not affect established connections.
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
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL outbound rule is blocking return traffic. — Even if inbound rules allow traffic, if the network ACL's outbound rule (stateless) does not allow return traffic (ephemeral ports), the connection will fail. Security groups are stateful and allow return traffic automatically.
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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