The answer is a missing route in the VPC route table for the RDS subnet. This is correct because even when security groups are properly configured to allow inbound traffic from the EC2 instance to the RDS database, the EC2 instance’s subnet route table must contain an explicit route to the RDS subnet’s CIDR block; without it, packets from the EC2 instance cannot reach the RDS database, and since security groups are stateful, the return traffic is automatically permitted. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how VPC routing, not just security groups or NACLs, governs inter-subnet connectivity—a common trap is assuming security group rules alone guarantee traffic flow. Remember that NACLs are stateless and require explicit inbound and outbound rules, but here the core issue is routing, not filtering. Memory tip: “Route before rules—packets need a path before they can be checked.”
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.
An application running on an EC2 instance is unable to connect to an Amazon RDS database in the same VPC. The security groups allow traffic from the EC2 instance. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route table does not have a route to the RDS subnet.
Option C is correct because security groups are stateful; if inbound is allowed, outbound response is automatically allowed. Option A is wrong because NACLs are stateless but inbound/outbound rules must allow traffic; however, security group statefulness means the issue is likely elsewhere. Option B is wrong because IAM permissions do not affect network connectivity. Option D is wrong because VPC flow logs are for monitoring, not connectivity.
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 IAM role attached to the EC2 instance does not have permissions to access RDS.
Why it's wrong here
IAM roles control AWS API access, not TCP connectivity.
✗
VPC Flow Logs are not enabled.
Why it's wrong here
Flow logs only provide visibility, they do not affect connectivity.
✓
The route table does not have a route to the RDS subnet.
Why this is correct
Without a route in the subnet's route table to the destination subnet, traffic cannot reach RDS.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
If NACLs were blocking, it would be a common issue, but security groups are stateful and should allow return traffic.
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 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 route table does not have a route to the RDS subnet. — Option C is correct because security groups are stateful; if inbound is allowed, outbound response is automatically allowed. Option A is wrong because NACLs are stateless but inbound/outbound rules must allow traffic; however, security group statefulness means the issue is likely elsewhere. Option B is wrong because IAM permissions do not affect network connectivity. Option D is wrong because VPC flow logs are for monitoring, not connectivity.
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.
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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