The correct answer is that the target instance’s security group does not allow HTTP traffic from the ALB’s security group. Even though the ALB’s security group correctly permits inbound HTTP from the VPC CIDR (10.0.0.0/16), health checks originate from the ALB’s private IPs, which fall within that CIDR—so the ALB side is fine. The failure occurs because the target’s security group must explicitly allow inbound traffic on port 80 from the ALB’s security group, not just from the VPC range. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding that ALB health checks are sent from the ALB’s own security group, not from a generic CIDR block. A common trap is focusing only on the ALB’s security group rules while ignoring the target’s inbound rules. Remember the mnemonic: “Health checks hit the target, not the ALB—so check the target’s SG first.”
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.
Network Topology
Refer to the exhibit. A security group is attached to an Application Load Balancer (ALB) that serves HTTPS traffic on port 443. Users can access the application via HTTPS. However, the ALB's health checks to targets on port 80 are failing. What is the reason?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the target instances does not allow HTTP traffic from the ALB's security group.
Option D is correct because the security group allows inbound traffic on port 80 only from the 10.0.0.0/16 CIDR, but the ALB's health check requests originate from the ALB's private IP, which is within the VPC CIDR (10.0.0.0/16). However, the target's security group must allow traffic from the ALB's security group. If the target's security group does not allow traffic from the ALB's security group, health checks fail. But the exhibit shows the ALB's security group, not the target's. The question might be tricky: The ALB's security group allows HTTP from VPC CIDR, which is fine. The issue might be that the target's security group does not allow traffic from the ALB's security group. However, based on the exhibit, the ALB's security group is correct. The most likely cause is that the target's security group does not allow inbound traffic from the ALB's security group. But since the exhibit is for the ALB, the answer should reference the target's security group. Option D states "The security group for the target instances does not allow HTTP traffic from the ALB's security group." This is correct because the health checks fail at the target level. Option A is wrong because the ALB's security group allows HTTPS from anywhere. Option B is wrong because the ALB's security group allows HTTP from VPC CIDR. Option C is wrong because health checks use HTTP, not HTTPS.
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 ALB's security group does not allow HTTPS traffic from the internet.
Why it's wrong here
Incorrect because HTTPS is allowed from 0.0.0.0/0.
✓
The security group for the target instances does not allow HTTP traffic from the ALB's security group.
Why this is correct
Correct because the ALB's health check requests are blocked by the target's security group.
The ALB's security group does not allow HTTP traffic from the target's IP range.
Why it's wrong here
Incorrect because HTTP is allowed from 10.0.0.0/16, which includes the VPC CIDR.
✗
The health check is configured to use HTTPS, but the target only supports HTTP.
Why it's wrong here
Incorrect because health checks are typically HTTP and the exhibit shows HTTP port 80 allowed.
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
Command / output trap
Incorrect because health checks are typically HTTP and the exhibit shows HTTP port 80 allowed.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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 security group for the target instances does not allow HTTP traffic from the ALB's security group. — Option D is correct because the security group allows inbound traffic on port 80 only from the 10.0.0.0/16 CIDR, but the ALB's health check requests originate from the ALB's private IP, which is within the VPC CIDR (10.0.0.0/16). However, the target's security group must allow traffic from the ALB's security group. If the target's security group does not allow traffic from the ALB's security group, health checks fail. But the exhibit shows the ALB's security group, not the target's. The question might be tricky: The ALB's security group allows HTTP from VPC CIDR, which is fine. The issue might be that the target's security group does not allow traffic from the ALB's security group. However, based on the exhibit, the ALB's security group is correct. The most likely cause is that the target's security group does not allow inbound traffic from the ALB's security group. But since the exhibit is for the ALB, the answer should reference the target's security group. Option D states "The security group for the target instances does not allow HTTP traffic from the ALB's security group." This is correct because the health checks fail at the target level. Option A is wrong because the ALB's security group allows HTTPS from anywhere. Option B is wrong because the ALB's security group allows HTTP from VPC CIDR. Option C is wrong because health checks use HTTP, not HTTPS.
What should I do if I get this SOA-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 SOA-C02 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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