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Question 609 of 1,746

Continuous Improvement for Existing Solutions →easyMultiple ChoiceObjective-mapped

Network ACL Blocking SSH

This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 DevOps engineer notices that an EC2 instance is running but is not reachable via SSH. The instance was launched with a security group that allows SSH from anywhere (0.0.0.0/0). 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 instance does not have a public IP address.
Why wrong: While an instance without a public IP cannot be reached from the internet, the question implies SSH from the internet is allowed by the security group, so the missing public IP is a possible cause but less common than a network ACL misconfiguration.
B
The instance is failing system status checks.
Why wrong: System status checks monitor the health of the underlying hardware; a failing check would cause the instance to stop or be impaired, but it wouldn't specifically block SSH traffic.
C
The security group is not attached to the instance.
Why wrong: Security groups are attached at launch; if the security group were not attached, SSH would still be blocked even if the rule exists, but the question states the security group allows SSH.
D
The subnet's network ACL is blocking inbound SSH traffic.
Network ACLs are stateless and can block traffic even if the security group allows it. If the inbound rule for port 22 is missing or denied in the network ACL, SSH connections will be dropped.

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 subnet's network ACL is blocking inbound SSH traffic.

The most likely cause is that the subnet's network ACL is blocking inbound SSH traffic. While the security group allows SSH from anywhere (0.0.0.0/0), network ACLs operate at the subnet level and can override security group rules. If the network ACL denies inbound traffic on port 22, SSH connections will be blocked. Option A is less likely because even without a public IP, the instance could be reached via private IP from within the VPC; the scenario implies external SSH access. Option B is incorrect because system status checks indicate underlying hardware issues, not connectivity problems. Option C is incorrect because the security group is attached by default when launching an instance, and the question confirms it allows SSH.

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 instance does not have a public IP address.
Why it's wrong here
While an instance without a public IP cannot be reached from the internet, the question implies SSH from the internet is allowed by the security group, so the missing public IP is a possible cause but less common than a network ACL misconfiguration.
✗
The instance is failing system status checks.
Why it's wrong here
System status checks monitor the health of the underlying hardware; a failing check would cause the instance to stop or be impaired, but it wouldn't specifically block SSH traffic.
✗
The security group is not attached to the instance.
Why it's wrong here
Security groups are attached at launch; if the security group were not attached, SSH would still be blocked even if the rule exists, but the question states the security group allows SSH.
✓
The subnet's network ACL is blocking inbound SSH traffic.
Why this is correct
Network ACLs are stateless and can block traffic even if the security group allows it. If the inbound rule for port 22 is missing or denied in the network ACL, SSH connections will be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.

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.

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

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FAQ

Questions learners often ask

What does this SAP-C02 question test?

Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The subnet's network ACL is blocking inbound SSH traffic. — The most likely cause is that the subnet's network ACL is blocking inbound SSH traffic. While the security group allows SSH from anywhere (0.0.0.0/0), network ACLs operate at the subnet level and can override security group rules. If the network ACL denies inbound traffic on port 22, SSH connections will be blocked. Option A is less likely because even without a public IP, the instance could be reached via private IP from within the VPC; the scenario implies external SSH access. Option B is incorrect because system status checks indicate underlying hardware issues, not connectivity problems. Option C is incorrect because the security group is attached by default when launching an instance, and the question confirms it allows SSH.

What should I do if I get this SAP-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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