← SOA-C02·Amazon Web Services

Question 853 of 1,546

Networking and Content Delivery →mediumMultiple SelectObjective-mapped

How to Allow EC2 in Public Subnet to Connect to RDS in Private Subnet via Security Groups

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.)

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.

Full breakdown with real-world context →

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.

The correct answers are A and D. To allow an EC2 instance in a public subnet to connect to an RDS for MySQL database in a private subnet, two steps are needed: (A) Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group. This permits the database to receive connections from the EC2 instance. (D) Ensure that the EC2 instance's security group allows outbound traffic to the RDS instance's security group on port 3306. This permits the EC2 instance to initiate the connection. Option B is incorrect because a NAT Gateway is for providing internet access to instances in private subnets, not for communication within the same VPC. Option C is incorrect because the RDS instance does not need a public IP; it communicates with the EC2 instance using private IPs within the VPC. Option E is incorrect because the route between subnets in the same VPC is automatically handled by the VPC's local route; no additional route is needed.

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.
Why it's wrong here
A NAT Gateway is used for private instances to access the internet, not for communication within the VPC.
✗
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.

Visual reference

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. — The correct answers are A and D. To allow an EC2 instance in a public subnet to connect to an RDS for MySQL database in a private subnet, two steps are needed: (A) Update the security group for the RDS instance to allow inbound traffic on port 3306 from the EC2 instance's security group. This permits the database to receive connections from the EC2 instance. (D) Ensure that the EC2 instance's security group allows outbound traffic to the RDS instance's security group on port 3306. This permits the EC2 instance to initiate the connection. Option B is incorrect because a NAT Gateway is for providing internet access to instances in private subnets, not for communication within the same VPC. Option C is incorrect because the RDS instance does not need a public IP; it communicates with the EC2 instance using private IPs within the VPC. Option E is incorrect because the route between subnets in the same VPC is automatically handled by the VPC's local route; no additional route is needed.

What should I do if I get this SOA-C02 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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