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Question 430 of 1,730

Monitoring and Troubleshooting →easyMultiple ChoiceObjective-mapped

DBS-C01 Monitoring and Troubleshooting Practice Question

This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 database specialist is trying to connect to an Amazon RDS for MySQL instance from an EC2 instance but receives a 'Connection timed out' error. The security group for the RDS instance allows inbound traffic on port 3306 from the security group of the EC2 instance. What should the specialist check next?

A
Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.
Network ACLs are stateless and must allow both inbound and outbound traffic.
B
Check that the RDS instance has a public DNS name and the EC2 instance can resolve it.
Why wrong: The error is a timeout, not a DNS resolution issue.
C
Ensure that the VPC has an internet gateway attached and the route table has a route to it.
Why wrong: If both instances are in the same VPC, internet gateway is not needed.
D
Verify that the security group for the EC2 instance allows outbound traffic on port 3306.
Why wrong: Outbound traffic is typically allowed by default; the issue is likely network ACL.

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

✓

Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.

Option A is correct because the 'Connection timed out' error suggests the packet is being dropped at the network layer, likely by a network ACL. Since the security group allows inbound on port 3306, the next step is to check the network ACL associated with the RDS instance's subnet. Network ACLs are stateless, so they must allow both inbound traffic on port 3306 and outbound traffic on ephemeral ports for the response. Option B is incorrect because a timed out error occurs before the TCP handshake completes, so DNS resolution is not the issue. Option C is incorrect because an internet gateway and public route are irrelevant if both instances are in the same VPC or connected privately. Option D is incorrect because the security group for the EC2 instance can be stateful and typically allows outbound traffic by default; the timeout is not caused by missing outbound rules on the EC2 side.

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.

✓
Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.
Why this is correct
Network ACLs are stateless and must allow both inbound and outbound traffic.
Related concept
CIDR notation defines the prefix length.
✗
Check that the RDS instance has a public DNS name and the EC2 instance can resolve it.
Why it's wrong here
The error is a timeout, not a DNS resolution issue.
✗
Ensure that the VPC has an internet gateway attached and the route table has a route to it.
Why it's wrong here
If both instances are in the same VPC, internet gateway is not needed.
✗
Verify that the security group for the EC2 instance allows outbound traffic on port 3306.
Why it's wrong here
Outbound traffic is typically allowed by default; the issue is likely network ACL.

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

Quick reference

OSI Model Reference

Layer	Name	PDU	Key Protocols / Devices
7	Application	Data	HTTP, HTTPS, DNS, SMTP, FTP, SSH
6	Presentation	Data	TLS / SSL, JPEG, ASCII encoding
5	Session	Data	NetBIOS, RPC, SIP
4	Transport	Segment / Datagram	TCP, UDP
3	Network	Packet	IP, ICMP, OSPF — Routers
2	Data Link	Frame	Ethernet, Wi-Fi, PPP — Switches, Bridges
1	Physical	Bits	Cables, NICs, Hubs, Repeaters

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 DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — 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 subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports. — Option A is correct because the 'Connection timed out' error suggests the packet is being dropped at the network layer, likely by a network ACL. Since the security group allows inbound on port 3306, the next step is to check the network ACL associated with the RDS instance's subnet. Network ACLs are stateless, so they must allow both inbound traffic on port 3306 and outbound traffic on ephemeral ports for the response. Option B is incorrect because a timed out error occurs before the TCP handshake completes, so DNS resolution is not the issue. Option C is incorrect because an internet gateway and public route are irrelevant if both instances are in the same VPC or connected privately. Option D is incorrect because the security group for the EC2 instance can be stateful and typically allows outbound traffic by default; the timeout is not caused by missing outbound rules on the EC2 side.

What should I do if I get this DBS-C01 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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