EC2 Not Receiving Traffic from ALB: Security Group Misconfiguration
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 an EC2 instance is not receiving traffic from an Application Load Balancer (ALB). The ALB is healthy and the target group shows the instance as healthy. The exhibit shows the network interface attached to the instance. What is the likely cause of the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance's security group does not allow inbound traffic from the ALB
The instance's security group must allow inbound traffic from the ALB. The exhibit shows the network interface attached to the instance, and the security group (sg-12345678) is likely the default security group, which typically does not allow HTTP/HTTPS traffic. Option C is correct because the security group acts as a virtual firewall for the instance. Options A, B, and D are incorrect: A) source/destination check is irrelevant for ALB traffic destined to the instance; B) the subnet type does not directly prevent ALB traffic as long as routing is correct; D) network ACLs are stateless and if inbound traffic is denied, outbound return traffic would also be affected, but the instance is not receiving traffic at all, indicating a security group issue.
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 source/destination check is enabled on the network interface
Why it's wrong here
Source/destination check is typically enabled and does not affect ALB traffic to the instance.
✗
The network interface is in a private subnet
Why it's wrong here
ALB ENIs can be in private subnets; this doesn't block traffic.
✓
The instance's security group does not allow inbound traffic from the ALB
Why this is correct
If the instance's security group blocks traffic from the ALB, the ALB will receive 503 errors.
The network ACL of the subnet denies inbound traffic
Why it's wrong here
Network ACLs are stateless; if inbound is denied, but the ALB ENI is in a different subnet, it's possible but less likely.
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.
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 instance's security group does not allow inbound traffic from the ALB — The instance's security group must allow inbound traffic from the ALB. The exhibit shows the network interface attached to the instance, and the security group (sg-12345678) is likely the default security group, which typically does not allow HTTP/HTTPS traffic. Option C is correct because the security group acts as a virtual firewall for the instance. Options A, B, and D are incorrect: A) source/destination check is irrelevant for ALB traffic destined to the instance; B) the subnet type does not directly prevent ALB traffic as long as routing is correct; D) network ACLs are stateless and if inbound traffic is denied, outbound return traffic would also be affected, but the instance is not receiving traffic at all, indicating a security group issue.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. A SysOps administrator runs the describe-target-health command and sees that an EC2 instance in the target group is unhealthy with a timeout error. What is the most likely cause?
easy
A.The target group is configured with an incorrect port
✓ B.The instance's security group does not allow traffic from the ALB on the health check port
C.The instance is in a private subnet without a NAT gateway
D.The instance does not have a route to the internet
Why B: Option B is correct because the health check is timing out, which indicates that the instance is not responding to health check requests on port 80. The security group must allow inbound traffic from the ALB on the health check port. Option A is wrong because the target group is configured with port 80, which matches. Option C is wrong because the route table is less likely to cause a timeout; it would cause unreachability. Option D is wrong because the instance is in a public subnet? Not necessarily; but the health check timeout is most often a security group issue.
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Question Discussion
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