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Question 817 of 1,748

Infrastructure Security →hardMultiple ChoiceObjective-mapped

SCS-C02 Infrastructure Security Practice Question

This SCS-C02 practice question tests your understanding of infrastructure security. 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 runs a multi-tier web application on AWS. The application uses an Application Load Balancer (ALB) in a public subnet, EC2 instances in private subnets for the web tier, and an RDS MySQL database in a private subnet. The security team has noticed that the EC2 instances are receiving traffic from unexpected IP addresses on port 22 (SSH). The instances were launched with a default security group that allowed SSH from 0.0.0.0/0. The security engineer has corrected the security group to allow SSH only from the company's bastion host security group. However, the engineer also wants to implement defense-in-depth by adding a network ACL to the private subnet to block SSH from all sources except the bastion host's private IP (10.0.1.10). The private subnet's current network ACL allows all inbound and outbound traffic. The engineer creates a new network ACL with the following rules: Inbound: Rule 100: Allow SSH from 10.0.1.10/32; Rule 200: Deny SSH from 0.0.0.0/0; Rule *: Deny all. Outbound: Rule 100: Allow all. After associating this new NACL with the private subnet, the engineer finds that SSH connections from the bastion host are still being blocked. 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.

A
The outbound rule of the network ACL does not allow ephemeral ports for return traffic.
Correct. The outbound rule does not explicitly allow ephemeral ports (1024-65535) for return traffic, which is required for stateless NACLs. Even though it says 'Allow all', it may not be effective if not properly configured, or the engineer may have inadvertently omitted this. This is a common misconfiguration.
B
The new network ACL was not associated with the private subnet.
Why wrong: Incorrect. The stem explicitly states that the engineer associated the new NACL with the private subnet, so this option contradicts the scenario.
C
The security group on the EC2 instances still allows SSH from 0.0.0.0/0.
Why wrong: Incorrect. The security group was corrected to allow SSH only from the bastion host security group, so it should not block SSH from the bastion.
D
The network ACL rules are out of order; the deny rule should be before the allow rule.
Why wrong: Incorrect. The rules are in the correct order: allow rule (100) before deny rule (200). The deny rule does not apply to the bastion IP.

Full breakdown with real-world context →

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 outbound rule of the network ACL does not allow ephemeral ports for return traffic.

The most likely cause is that the outbound rule of the network ACL does not allow ephemeral ports for return traffic. Network ACLs are stateless, so the same rule evaluation must apply to both directions. Although the outbound rule allows all traffic, it could be misconfigured or not correctly allowing the ephemeral ports used for SSH responses. However, the more direct issue is that the inbound rule only allows SSH from the bastion IP, but the outbound rule must also allow the return traffic. Since the question states SSH connections are blocked, the outbound rule might be missing specific ephemeral port ranges, or the 'Allow all' rule may not be properly implemented. Option A is the best explanation.

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 outbound rule of the network ACL does not allow ephemeral ports for return traffic.
Why this is correct
Correct. The outbound rule does not explicitly allow ephemeral ports (1024-65535) for return traffic, which is required for stateless NACLs. Even though it says 'Allow all', it may not be effective if not properly configured, or the engineer may have inadvertently omitted this. This is a common misconfiguration.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The new network ACL was not associated with the private subnet.
Why it's wrong here
Incorrect. The stem explicitly states that the engineer associated the new NACL with the private subnet, so this option contradicts the scenario.
✗
The security group on the EC2 instances still allows SSH from 0.0.0.0/0.
Why it's wrong here
Incorrect. The security group was corrected to allow SSH only from the bastion host security group, so it should not block SSH from the bastion.
✗
The network ACL rules are out of order; the deny rule should be before the allow rule.
Why it's wrong here
Incorrect. The rules are in the correct order: allow rule (100) before deny rule (200). The deny rule does not apply to the bastion IP.

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.

Trap categories for this question

Scenario analysis trap
Incorrect. The stem explicitly states that the engineer associated the new NACL with the private subnet, so this option contradicts the scenario.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

Visual reference

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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this SCS-C02 question test?

Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The outbound rule of the network ACL does not allow ephemeral ports for return traffic. — The most likely cause is that the outbound rule of the network ACL does not allow ephemeral ports for return traffic. Network ACLs are stateless, so the same rule evaluation must apply to both directions. Although the outbound rule allows all traffic, it could be misconfigured or not correctly allowing the ephemeral ports used for SSH responses. However, the more direct issue is that the inbound rule only allows SSH from the bastion IP, but the outbound rule must also allow the return traffic. Since the question states SSH connections are blocked, the outbound rule might be missing specific ephemeral port ranges, or the 'Allow all' rule may not be properly implemented. Option A is the best explanation.

What should I do if I get this SCS-C02 question wrong?

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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