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Question 1,072 of 1,546

Networking and Content Delivery →hardMultiple ChoiceObjective-mapped

SOA-C02 Networking and Content Delivery Practice Question

This SOA-C02 practice question tests your understanding of networking and content delivery. 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 SysOps administrator notices that traffic from an Application Load Balancer to EC2 instances is failing intermittently. Security groups for the instances allow traffic from the ALB security group on port 80. The ALB target group 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.

A
The network ACL for the instance's subnet is blocking inbound traffic from the ALB's subnet.
Network ACLs are stateless; if they deny inbound health check traffic from the ALB subnet, health checks will fail.
B
The instance security group does not allow outbound traffic to the ALB.
Why wrong: Security groups are stateful; inbound rule implicitly allows return traffic.
C
The ALB security group does not allow outbound traffic to the instances.
Why wrong: The ALB security group's outbound rules are not the issue; the instance security group must allow inbound from the ALB.
D
The ALB is in a public subnet without an internet gateway.
Why wrong: The ALB can communicate with instances in private subnets via the VPC; an internet gateway is not required for health checks.

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 network ACL for the instance's subnet is blocking inbound traffic from the ALB's subnet.

Option A is correct. Network ACLs are stateless, so the subnet's NACL must explicitly allow inbound traffic from the ALB's subnet CIDR. If it does not, health check traffic from the ALB will be blocked. Option B is incorrect because the instance's security group allows inbound traffic from the ALB security group, and security groups are stateful, so return traffic is allowed automatically. Option C is incorrect because the ALB security group does not need an outbound rule; security groups are stateful, and the ALB initiates connections to the instances. Option D is incorrect because the ALB can communicate within the VPC regardless of whether it is in a public subnet; an internet gateway is only needed for internet traffic, not for internal ALB-to-instance traffic.

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 for the instance's subnet is blocking inbound traffic from the ALB's subnet.
Why this is correct
Network ACLs are stateless; if they deny inbound health check traffic from the ALB subnet, health checks will fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The instance security group does not allow outbound traffic to the ALB.
Why it's wrong here
Security groups are stateful; inbound rule implicitly allows return traffic.
✗
The ALB security group does not allow outbound traffic to the instances.
Why it's wrong here
The ALB security group's outbound rules are not the issue; the instance security group must allow inbound from the ALB.
✗
The ALB is in a public subnet without an internet gateway.
Why it's wrong here
The ALB can communicate with instances in private subnets via the VPC; an internet gateway is not required for health checks.

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 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.

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

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FAQ

Questions learners often ask

What does this SOA-C02 question test?

Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The network ACL for the instance's subnet is blocking inbound traffic from the ALB's subnet. — Option A is correct. Network ACLs are stateless, so the subnet's NACL must explicitly allow inbound traffic from the ALB's subnet CIDR. If it does not, health check traffic from the ALB will be blocked. Option B is incorrect because the instance's security group allows inbound traffic from the ALB security group, and security groups are stateful, so return traffic is allowed automatically. Option C is incorrect because the ALB security group does not need an outbound rule; security groups are stateful, and the ALB initiates connections to the instances. Option D is incorrect because the ALB can communicate within the VPC regardless of whether it is in a public subnet; an internet gateway is only needed for internet traffic, not for internal ALB-to-instance traffic.

What should I do if I get this SOA-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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