- A
Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group.
This allows the EC2 instance to connect to the database.
- B
Deploy a NAT Gateway in the public subnet to allow the EC2 instance to communicate with the RDS instance.
Why wrong: A NAT Gateway is used for private instances to access the internet, not for communication within the VPC.
- C
Assign a public IP address to the RDS instance.
Why wrong: RDS in a private subnet should not have a public IP; it is accessed via private IP within the VPC.
- D
Ensure that the EC2 instance's security group allows outbound traffic to the RDS instance's security group on port 3306.
Outbound rules must allow the connection to the database; by default, security groups allow all outbound traffic, but if custom rules are set, they must permit it.
- E
Add a route to the public subnet's route table for the RDS subnet CIDR.
Why wrong: The route table for the public subnet already has local routes for the VPC CIDR, so traffic to the private subnet is automatically routed.
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. An Amazon RDS for MySQL database is deployed in the private subnet. Which TWO steps are required to allow an EC2 instance in the public subnet to connect to the database? (Choose two.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group.
Options B and D are correct: The security group for the RDS instance must allow inbound MySQL traffic (port 3306) from the EC2 instance (or its security group). The route table for the private subnet does not need a NAT Gateway because the connection is initiated from the public subnet; however, the public subnet instance needs a route to the private subnet, which is automatically there because both are in the same VPC. Option A is wrong because the RDS instance does not need a public IP; it can be accessed from within the VPC using its private IP. Option C is wrong because the EC2 instance already has a public IP or can be accessed via internet, but the route to the private subnet is implicit. Option E is wrong because a NAT Gateway is for outbound internet access from private subnets, not for inbound connections.
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.
- ✓
Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group.
Why this is correct
This allows the EC2 instance to connect to the database.
Related concept
CIDR notation defines the prefix length.
- ✗
Deploy a NAT Gateway in the public subnet to allow the EC2 instance to communicate with the RDS instance.
- ✗
Assign a public IP address to the RDS instance.
Why it's wrong here
RDS in a private subnet should not have a public IP; it is accessed via private IP within the VPC.
- ✓
Ensure that the EC2 instance's security group allows outbound traffic to the RDS instance's security group on port 3306.
Why this is correct
Outbound rules must allow the connection to the database; by default, security groups allow all outbound traffic, but if custom rules are set, they must permit it.
Related concept
CIDR notation defines the prefix length.
- ✗
Add a route to the public subnet's route table for the RDS subnet CIDR.
Why it's wrong here
The route table for the public subnet already has local routes for the VPC CIDR, so traffic to the private subnet is automatically routed.
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.
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
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: Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group. — Options B and D are correct: The security group for the RDS instance must allow inbound MySQL traffic (port 3306) from the EC2 instance (or its security group). The route table for the private subnet does not need a NAT Gateway because the connection is initiated from the public subnet; however, the public subnet instance needs a route to the private subnet, which is automatically there because both are in the same VPC. Option A is wrong because the RDS instance does not need a public IP; it can be accessed from within the VPC using its private IP. Option C is wrong because the EC2 instance already has a public IP or can be accessed via internet, but the route to the private subnet is implicit. Option E is wrong because a NAT Gateway is for outbound internet access from private subnets, not for inbound connections.
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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Last reviewed: Jun 20, 2026
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