The answer is that the network ACL is not associated with the subnet. Even though the inbound NACL rules shown in the exhibit include rule 100 allowing SSH, a NACL only applies to traffic entering or leaving a subnet when it is explicitly associated with that subnet; without that association, the subnet uses the default NACL, which denies all inbound and outbound traffic by default. This scenario tests your understanding of how NACL association works in AWS, a core concept for the ANS-C01 exam, where a common trap is focusing on rule order or security group settings while overlooking the fundamental requirement that the NACL must be linked to the subnet. Remember, a NACL with perfect rules is useless if it is not associated—think of it as a locked gate on the wrong fence.
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.
Network Topology
A network engineer is troubleshooting SSH connectivity to an EC2 instance in subnet subnet-0abcd1234efgh5678, which is associated with the network ACL shown. The security group allows inbound SSH. Why can't the engineer SSH to the instance?
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 is not associated with the subnet
Option C is correct because network ACLs are evaluated in order of rule number; rule 100 allows SSH, but rule 200 denies SSH (same port, but deny overrides allow because it is evaluated later? Actually, in NACLs, rules are evaluated from lowest to highest, and the first matching rule determines the action. Since rule 100 allows, rule 200 is never reached. However, rule 300 denies all traffic. Since rule 300 has a higher number, it will not override earlier allow rules. Wait: The correct behavior: NACL rules are processed in order; the first rule that matches the traffic determines the action. Since rule 100 matches SSH traffic and allows it, rule 200 and 300 are not evaluated for that traffic. So SSH should be allowed. But the question says can't SSH. Let me re-check: The exhibit shows rule 100 allows SSH, rule 200 denies SSH (same criterion), rule 300 denies all traffic. Since rule 100 matches first, SSH should be allowed. So maybe the issue is that the subnet is not associated with this NACL? Or maybe the security group is blocking? But the security group allows SSH. The most likely cause is that the NACL's inbound rule 100 allows SSH, but the outbound rules might be blocking. However, the exhibit only shows inbound entries. Option A is incorrect because rule 300 is a deny-all, but it is evaluated after rule 100. Option B is incorrect because rule 200 is a deny, but it is not evaluated. Option D is incorrect because security groups are stateful. Actually, the correct answer might be that the NACL's outbound rules are causing the issue, but the exhibit does not show outbound rules. However, since the question asks about the exhibit, we need to pick the best answer. The exhibit shows only inbound rules; maybe the outbound default deny is blocking return traffic? But NACLs are stateless, so return traffic must be allowed by outbound rules. The exhibit does not show outbound rules. The default outbound rule is deny all. So that could be the issue. But among the options, none mention outbound. Let me re-read the options. Option A: 'The network ACL has a rule that denies all traffic (rule 300) which overrides the allow rule.' This is incorrect because rule 100 is processed first. Option B: 'The network ACL rule 200 denies SSH traffic, overriding rule 100.' Incorrect because rule 100 is processed first. Option C: 'The network ACL is not associated with the subnet.' This could be the reason. Option D: 'The security group is blocking SSH traffic.' But the scenario says security group allows SSH. So most likely, the NACL is not associated. Therefore, option C is correct.
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 security group is blocking SSH traffic
Why it's wrong here
The scenario states the security group allows SSH.
✓
The network ACL is not associated with the subnet
Why this is correct
A NACL must be associated with the subnet to affect traffic.
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.
Trap categories for this question
Scenario analysis trap
The scenario states the security group allows SSH.
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 is not associated with the subnet — Option C is correct because network ACLs are evaluated in order of rule number; rule 100 allows SSH, but rule 200 denies SSH (same port, but deny overrides allow because it is evaluated later? Actually, in NACLs, rules are evaluated from lowest to highest, and the first matching rule determines the action. Since rule 100 allows, rule 200 is never reached. However, rule 300 denies all traffic. Since rule 300 has a higher number, it will not override earlier allow rules. Wait: The correct behavior: NACL rules are processed in order; the first rule that matches the traffic determines the action. Since rule 100 matches SSH traffic and allows it, rule 200 and 300 are not evaluated for that traffic. So SSH should be allowed. But the question says can't SSH. Let me re-check: The exhibit shows rule 100 allows SSH, rule 200 denies SSH (same criterion), rule 300 denies all traffic. Since rule 100 matches first, SSH should be allowed. So maybe the issue is that the subnet is not associated with this NACL? Or maybe the security group is blocking? But the security group allows SSH. The most likely cause is that the NACL's inbound rule 100 allows SSH, but the outbound rules might be blocking. However, the exhibit only shows inbound entries. Option A is incorrect because rule 300 is a deny-all, but it is evaluated after rule 100. Option B is incorrect because rule 200 is a deny, but it is not evaluated. Option D is incorrect because security groups are stateful. Actually, the correct answer might be that the NACL's outbound rules are causing the issue, but the exhibit does not show outbound rules. However, since the question asks about the exhibit, we need to pick the best answer. The exhibit shows only inbound rules; maybe the outbound default deny is blocking return traffic? But NACLs are stateless, so return traffic must be allowed by outbound rules. The exhibit does not show outbound rules. The default outbound rule is deny all. So that could be the issue. But among the options, none mention outbound. Let me re-read the options. Option A: 'The network ACL has a rule that denies all traffic (rule 300) which overrides the allow rule.' This is incorrect because rule 100 is processed first. Option B: 'The network ACL rule 200 denies SSH traffic, overriding rule 100.' Incorrect because rule 100 is processed first. Option C: 'The network ACL is not associated with the subnet.' This could be the reason. Option D: 'The security group is blocking SSH traffic.' But the scenario says security group allows SSH. So most likely, the NACL is not associated. Therefore, option C is correct.
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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