The correct answer requires a security group rule on the database allowing inbound traffic from the EC2 instance’s security group, and a network ACL (NACL) on the private subnet allowing inbound traffic from the public subnet. Security groups act as a virtual firewall at the instance level, supporting stateful allow rules based on source security group IDs, which is why the database’s security group must explicitly permit traffic from the public EC2 instance. Network ACLs operate at the subnet level as stateless filters, so the private subnet’s NACL must also allow inbound traffic from the public subnet’s CIDR range to ensure connectivity. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of layered security controls for VPC subnet connectivity, with a common trap being to confuse stateful security groups with stateless NACLs. A key memory tip is: security groups are instance-level and stateful—they track return traffic automatically—while NACLs are subnet-level and stateless, requiring explicit inbound and outbound rules.
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company has a VPC with a public subnet and a private subnet. The private subnet hosts a database. Which TWO components are required to allow an EC2 instance in the public subnet to connect to the database?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A network ACL rule on the private subnet allowing inbound traffic from the public subnet CIDR.
Options A and D are correct. The security group of the database must allow inbound traffic from the EC2 instance's security group or IP, and the network ACL of the private subnet must allow inbound traffic from the public subnet. Option B is incorrect because a NATGateway is for outbound internet, not inbound. Option C is incorrect because an Internet Gateway is for internet traffic, not VPC internal traffic. Option E is incorrect because a VPC Endpoint is for accessing AWS services, not EC2-to-RDS.
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.
✗
A NAT Gateway in the public subnet.
Why it's wrong here
NATGateway enables outbound internet from private subnets, not inbound.
✓
A network ACL rule on the private subnet allowing inbound traffic from the public subnet CIDR.
Why this is correct
Network ACLs provide stateless filtering at the subnet level.
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: A network ACL rule on the private subnet allowing inbound traffic from the public subnet CIDR. — Options A and D are correct. The security group of the database must allow inbound traffic from the EC2 instance's security group or IP, and the network ACL of the private subnet must allow inbound traffic from the public subnet. Option B is incorrect because a NAT Gateway is for outbound internet, not inbound. Option C is incorrect because an Internet Gateway is for internet traffic, not VPC internal traffic. Option E is incorrect because a VPC Endpoint is for accessing AWS services, not EC2-to-RDS.
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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