The answer is to verify the route table in the EC2 instance's subnet has a route to the RDS subnet and to check the network ACL (NACL) rules on both subnets. This is correct because even with properly configured security groups, traffic can still fail if the subnet’s route table lacks a route to the destination subnet’s CIDR block, or if a NACL’s stateless rules explicitly deny the traffic. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this question tests your ability to troubleshoot VPC connectivity between EC2 and RDS by isolating layer 3 and layer 4 issues before blaming security groups. A common trap is assuming security groups alone control all traffic, but NACLs act as a subnet-level firewall that must allow both inbound and outbound traffic. Remember the memory tip: “Routes direct, NACLs filter — security groups are just the last line of defense.”
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 is troubleshooting a connectivity issue from an EC2 instance to an RDS database in the same VPC. The security groups are configured correctly. Which TWO steps should the administrator take to diagnose the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the network ACL associated with the subnets for both instances
Options B and D are correct. Checking route tables ensures the subnet routes are correct, and verifying NACL rules ensures they are not blocking traffic. Option A is wrong because flow logs do not affect connectivity. Option C is wrong because IAM permissions do not control network connectivity. Option E is wrong because subnet CIDR is unrelated to 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.
✗
Verify the CIDR block of the VPC
Why it's wrong here
CIDR block is not the cause of connectivity issues between instances in the same VPC.
✗
Enable VPC Flow Logs to capture traffic
Why it's wrong here
Flow logs help analyze traffic but do not diagnose the root cause directly.
✗
Check the IAM policy attached to the EC2 instance
Why it's wrong here
IAM policies do not affect network connectivity.
✓
Check the network ACL associated with the subnets for both instances
Why this is correct
NACLs are stateless and may block traffic if rules are misconfigured.
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: Check the network ACL associated with the subnets for both instances — Options B and D are correct. Checking route tables ensures the subnet routes are correct, and verifying NACL rules ensures they are not blocking traffic. Option A is wrong because flow logs do not affect connectivity. Option C is wrong because IAM permissions do not control network connectivity. Option E is wrong because subnet CIDR is unrelated to 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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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