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.
Exhibit
VPC: vpc-12345
Subnet: subnet-67890 (10.0.1.0/24, us-east-1a)
Network ACL: acl-abcde (associated with subnet-67890)
Inbound rules:
Rule 100: HTTP (80) | Source: 10.0.0.0/16 | ALLOW
Rule 200: HTTPS (443) | Source: 10.0.0.0/16 | ALLOW
Rule *: ALL Traffic | Source: 0.0.0.0/0 | DENY
Outbound rules:
Rule 100: HTTP (80) | Destination: 10.0.0.0/16 | ALLOW
Rule 200: HTTPS (443) | Destination: 10.0.0.0/16 | ALLOW
Rule *: ALL Traffic | Destination: 0.0.0.0/0 | DENY
Security Group: sg-99999 (attached to EC2 instance in subnet-67890)
Inbound: HTTP (80) Source: 0.0.0.0/0
Outbound: ALL Traffic Destination: 0.0.0.0/0
An internet-facing Application Load Balancer (ALB) in a public subnet sends traffic to the EC2 instance on port 80.
Refer to the exhibit. An application running on EC2 behind an ALB is unreachable from the internet. The ALB health checks are failing. 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.
VPC: vpc-12345
Subnet: subnet-67890 (10.0.1.0/24, us-east-1a)
Network ACL: acl-abcde (associated with subnet-67890)
Inbound rules:
Rule 100: HTTP (80) | Source: 10.0.0.0/16 | ALLOW
Rule 200: HTTPS (443) | Source: 10.0.0.0/16 | ALLOW
Rule *: ALL Traffic | Source: 0.0.0.0/0 | DENY
Outbound rules:
Rule 100: HTTP (80) | Destination: 10.0.0.0/16 | ALLOW
Rule 200: HTTPS (443) | Destination: 10.0.0.0/16 | ALLOW
Rule *: ALL Traffic | Destination: 0.0.0.0/0 | DENY
Security Group: sg-99999 (attached to EC2 instance in subnet-67890)
Inbound: HTTP (80) Source: 0.0.0.0/0
Outbound: ALL Traffic Destination: 0.0.0.0/0
An internet-facing Application Load Balancer (ALB) in a public subnet sends traffic to the EC2 instance on port 80.
A
The security group inbound rule allows HTTP from 0.0.0.0/0, but it should only allow traffic from the ALB's security group.
Why wrong: While best practice, allowing 0.0.0.0/0 does not block ALB traffic; the issue is network ACL.
B
The network ACL outbound rules deny all traffic, so the instance cannot send responses back to the ALB.
Why wrong: Outbound rules affect traffic leaving the subnet. The ALB initiates the connection; the response is also inbound to the ALB. But the outbound rule for the instance subnet would affect the response? Actually, the response is outbound from the instance subnet to the ALB subnet. The outbound rules allow only to 10.0.0.0/16, which may not include the ALB's subnet if it's outside that range. However, the primary issue is likely inbound.
C
The security group outbound rule allows all traffic, which is too permissive and causes the ALB to reject the instance as unhealthy.
Why wrong: Security groups are stateful; outbound rules do not affect health checks. The ALB health check is initiated by the ALB, and the response is allowed by the security group inbound rule.
D
The network ACL inbound rule only allows traffic from 10.0.0.0/16 on ports 80 and 443, but the ALB is in a different subnet (public subnet) with a different CIDR (e.g., 10.0.2.0/24). The ALB's traffic to the instance must come from its private IP, which is in the same VPC but may not be within 10.0.0.0/16 if the VPC CIDR is not /16.
The network ACL allows only from 10.0.0.0/16. If the VPC CIDR is, for example, 10.0.0.0/20, the ALB's private IP could be outside that range. Also, the ALB's traffic originates from its private IP, so if the subnet CIDR is not within 10.0.0.0/16, traffic is denied.
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 inbound rule only allows traffic from 10.0.0.0/16 on ports 80 and 443, but the ALB is in a different subnet (public subnet) with a different CIDR (e.g., 10.0.2.0/24). The ALB's traffic to the instance must come from its private IP, which is in the same VPC but may not be within 10.0.0.0/16 if the VPC CIDR is not /16.
The network ACL inbound rules only allow traffic from 10.0.0.0/16 on HTTP/HTTPS. If the ALB's private IP is not within that CIDR (e.g., VPC is 10.0.0.0/20, ALB subnet is 10.0.2.0/24, which is within 10.0.0.0/16 if /16 covers all 10.0.x.x. Actually, 10.0.0.0/16 covers 10.0.0.0 to 10.0.255.255, so 10.0.2.0/24 is within. However, if the VPC CIDR is different, it might not. The most common mistake is network ACL not allowing the ALB's source IP. Option B 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 inbound rule allows HTTP from 0.0.0.0/0, but it should only allow traffic from the ALB's security group.
Why it's wrong here
While best practice, allowing 0.0.0.0/0 does not block ALB traffic; the issue is network ACL.
✗
The network ACL outbound rules deny all traffic, so the instance cannot send responses back to the ALB.
Why it's wrong here
Outbound rules affect traffic leaving the subnet. The ALB initiates the connection; the response is also inbound to the ALB. But the outbound rule for the instance subnet would affect the response? Actually, the response is outbound from the instance subnet to the ALB subnet. The outbound rules allow only to 10.0.0.0/16, which may not include the ALB's subnet if it's outside that range. However, the primary issue is likely inbound.
✗
The security group outbound rule allows all traffic, which is too permissive and causes the ALB to reject the instance as unhealthy.
Why it's wrong here
Security groups are stateful; outbound rules do not affect health checks. The ALB health check is initiated by the ALB, and the response is allowed by the security group inbound rule.
✓
The network ACL inbound rule only allows traffic from 10.0.0.0/16 on ports 80 and 443, but the ALB is in a different subnet (public subnet) with a different CIDR (e.g., 10.0.2.0/24). The ALB's traffic to the instance must come from its private IP, which is in the same VPC but may not be within 10.0.0.0/16 if the VPC CIDR is not /16.
Why this is correct
The network ACL allows only from 10.0.0.0/16. If the VPC CIDR is, for example, 10.0.0.0/20, the ALB's private IP could be outside that range. Also, the ALB's traffic originates from its private IP, so if the subnet CIDR is not within 10.0.0.0/16, traffic is denied.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
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 network ACL inbound rule only allows traffic from 10.0.0.0/16 on ports 80 and 443, but the ALB is in a different subnet (public subnet) with a different CIDR (e.g., 10.0.2.0/24). The ALB's traffic to the instance must come from its private IP, which is in the same VPC but may not be within 10.0.0.0/16 if the VPC CIDR is not /16. — The network ACL inbound rules only allow traffic from 10.0.0.0/16 on HTTP/HTTPS. If the ALB's private IP is not within that CIDR (e.g., VPC is 10.0.0.0/20, ALB subnet is 10.0.2.0/24, which is within 10.0.0.0/16 if /16 covers all 10.0.x.x. Actually, 10.0.0.0/16 covers 10.0.0.0 to 10.0.255.255, so 10.0.2.0/24 is within. However, if the VPC CIDR is different, it might not. The most common mistake is network ACL not allowing the ALB's source IP. Option B is correct.
What should I do if I get this SCS-C02 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 SCS-C02 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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