The correct next step is to verify that the VPC subnets have a route to a NAT Gateway or Internet Gateway. This is because ECS Fargate tasks must pull container images from Amazon ECR or Docker Hub during startup, which requires outbound internet access from the task’s elastic network interface. Without a route to a NAT Gateway (for private subnets) or an Internet Gateway (for public subnets), the task cannot download the image and will fail to start, even if the security group allows inbound traffic on port 8080. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of Fargate networking dependencies versus common misdiagnoses like CPU/memory limits or security group rules—a frequent trap is assuming inbound rules are the issue when the real problem is missing outbound connectivity. Remember the mnemonic: “Fargate fails, check the gate”—meaning always verify the route to the NAT or Internet Gateway first when tasks won’t launch.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 DevOps engineer receives an alert that an Amazon ECS service is failing to start tasks. The service uses the Fargate launch type. The task definition includes a container that requires port 8080. The security group associated with the service allows inbound traffic on port 8080. What should the engineer check NEXT?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the VPC subnets have a route to a NAT Gateway or Internet Gateway.
Option B is correct because Fargate tasks require a VPC and subnets with outbound internet access (NATGateway or Internet Gateway) to pull images from ECR or Docker Hub. Option A is wrong because CPU/memory limits would cause tasks to fail with resource errors, not prevent starting. Option C is wrong because the security group is already configured. Option D is wrong because the task definition is already defined.
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.
✓
Verify that the VPC subnets have a route to a NAT Gateway or Internet Gateway.
Why this is correct
Fargate tasks need outbound internet access to pull images.
Confirm that the task definition's container image exists in ECR.
Why it's wrong here
The image location is specified in the task definition; it should exist.
✗
Check if the task definition has sufficient CPU and memory allocated.
Why it's wrong here
Resource limits affect running tasks, not starting them.
✗
Review the security group rules for outbound traffic.
Why it's wrong here
Outbound traffic is typically allowed by default.
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.
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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Incident and Event Response — This question tests Incident and Event Response — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Verify that the VPC subnets have a route to a NAT Gateway or Internet Gateway. — Option B is correct because Fargate tasks require a VPC and subnets with outbound internet access (NAT Gateway or Internet Gateway) to pull images from ECR or Docker Hub. Option A is wrong because CPU/memory limits would cause tasks to fail with resource errors, not prevent starting. Option C is wrong because the security group is already configured. Option D is wrong because the task definition is already defined.
What should I do if I get this DOP-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 DOP-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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Question Discussion
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