The answer is a missing outbound rule for port 80 in the network ACL. Although the NACL’s inbound rule correctly allows HTTP from 0.0.0.0/0 and the outbound rule permits ephemeral ports 1024-65535, the stateless nature of network ACLs means that every packet crossing the subnet boundary is evaluated independently. When the web server responds to a client’s HTTP request, it sends traffic from its own port 80 to the client’s ephemeral port; the outbound rule must explicitly allow this source port 80 traffic, not just the ephemeral range. This scenario tests your understanding of the fundamental difference between stateful security groups and stateless NACLs on the ANS-C01 exam—a classic trap where engineers assume the security group’s stateful return traffic handling applies to the NACL. Remember the mnemonic: “Stateless needs both ways; stateful remembers the days.”
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 single public subnet. The subnet has a web server that needs to be accessible over the internet. The security group for the web server allows inbound HTTP (port 80) from 0.0.0.0/0. The network ACL for the subnet allows inbound HTTP from 0.0.0.0/0 and outbound traffic on ports 1024-65535 to 0.0.0.0/0. The internet gateway is attached to the VPC, and the route table has a route to the internet gateway for 0.0.0.0/0. The web server has a public IP address. However, users cannot access the web server. The engineer verifies that the web server is running and listening on port 80. 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 network ACL's outbound rule does not allow traffic from port 80 to the internet.
Although the security group allows inbound HTTP, the network ACL's inbound rule only allows HTTP. However, the network ACL is stateless, so for a user to access the web server, the inbound rule must allow the HTTP traffic, and the outbound rule must allow return traffic. The outbound rule allows ephemeral ports, which is correct. However, the network ACL's default deny all inbound could be blocking if the rule order is wrong. But the question says the NACL allows inbound HTTP. The likely issue is that the security group is stateful and allows return traffic, but the network ACL is not stateful, and if the inbound rule does not allow the initial HTTP, it would fail. But it does allow inbound HTTP. So the issue might be that the security group's outbound rule is blocking the return traffic? No, security group is stateful. The most common mistake is that the network ACL's inbound rule does not allow the HTTP traffic from the internet because the rule number is higher than a deny rule. However, the question states the NACL allows inbound HTTP. Another common issue is that the web server's OS firewall is blocking. But the engineer verified the server is listening. The likely cause is that the security group is attached to the wrong ENI or the web server is in a private subnet. But the question says public subnet. The answer: the network ACL's outbound rule might be missing the return traffic for the HTTP response. The response uses ephemeral ports, but the outbound rule allows all outbound on ephemeral ports, so that's fine. Wait: the outbound rule allows traffic to 0.0.0.0/0 on ports 1024-65535, but the HTTP response comes from the server's port 80 to the client's ephemeral port, so the outbound traffic is from port 80 to the client's ephemeral port. The NACL outbound rule should allow traffic from the server's port 80 to the client's port. The outbound rule only allows source port 1024-65535, not port 80. That is the issue! The outbound rule must allow traffic from the server's port (80) to the client's ephemeral port. So the missing outbound rule for port 80 causes the response to be dropped.
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 network ACL's outbound rule does not allow traffic from port 80 to the internet.
Why this is correct
The NACL outbound rule only allows source ports 1024-65535, but the HTTP response comes from port 80, so it is blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table for the subnet does not have a route to the internet gateway.
Why it's wrong here
The question states the route table has a route to the IGW.
✗
The internet gateway is not properly attached to the VPC.
Why it's wrong here
If the IGW were not attached, no traffic would reach the server.
✗
The security group's outbound rule is blocking the HTTP response.
Why it's wrong here
Security groups are stateful, so 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's outbound rule does not allow traffic from port 80 to the internet. — Although the security group allows inbound HTTP, the network ACL's inbound rule only allows HTTP. However, the network ACL is stateless, so for a user to access the web server, the inbound rule must allow the HTTP traffic, and the outbound rule must allow return traffic. The outbound rule allows ephemeral ports, which is correct. However, the network ACL's default deny all inbound could be blocking if the rule order is wrong. But the question says the NACL allows inbound HTTP. The likely issue is that the security group is stateful and allows return traffic, but the network ACL is not stateful, and if the inbound rule does not allow the initial HTTP, it would fail. But it does allow inbound HTTP. So the issue might be that the security group's outbound rule is blocking the return traffic? No, security group is stateful. The most common mistake is that the network ACL's inbound rule does not allow the HTTP traffic from the internet because the rule number is higher than a deny rule. However, the question states the NACL allows inbound HTTP. Another common issue is that the web server's OS firewall is blocking. But the engineer verified the server is listening. The likely cause is that the security group is attached to the wrong ENI or the web server is in a private subnet. But the question says public subnet. The answer: the network ACL's outbound rule might be missing the return traffic for the HTTP response. The response uses ephemeral ports, but the outbound rule allows all outbound on ephemeral ports, so that's fine. Wait: the outbound rule allows traffic to 0.0.0.0/0 on ports 1024-65535, but the HTTP response comes from the server's port 80 to the client's ephemeral port, so the outbound traffic is from port 80 to the client's ephemeral port. The NACL outbound rule should allow traffic from the server's port 80 to the client's port. The outbound rule only allows source port 1024-65535, not port 80. That is the issue! The outbound rule must allow traffic from the server's port (80) to the client's ephemeral port. So the missing outbound rule for port 80 causes the response to be dropped.
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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