- A
The network cable is faulty.
Why wrong: If the cable were faulty, ping would also fail.
- B
The app server's firewall is blocking outgoing connections to port 3306.
Why wrong: This would cause telnet to fail, but ping would also fail if outbound ICMP were blocked? Actually ping might work if ICMP is allowed, but the question does not indicate firewall on app server. It's less likely.
- C
The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped.
Even though ping works due to ICMP being handled differently, TCP connections require consistent routing. A missing return route would cause TCP SYNs to be sent but ACKs never received, resulting in connection timeout or refused.
- D
The database service is only listening on the loopback interface (127.0.0.1).
Why wrong: The stem says MySQL is listening on 0.0.0.0:3306, so this is false.
LFCS Networking Practice Question
This LFCS practice question tests your understanding of networking. 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.
You are a Linux administrator for a small company. A developer has deployed a web application on a server with IP 192.168.1.50. The application needs to connect to a database server at 10.0.0.100 on TCP port 3306. Both servers are on the same physical network but different subnets (192.168.1.0/24 and 10.0.0.0/24) connected by a router. The default gateway for the app server is 192.168.1.1, and for the DB server is 10.0.0.1. You have verified that the app server can ping the DB server by IP address successfully. However, the application fails to connect to the database. You have used telnet from the app server to test connectivity on port 3306 and it fails (connection refused). On the DB server, you check that the MySQL service is listening on 0.0.0.0:3306 and that the local firewall (firewalld) allows incoming connections on port 3306. What is the most likely cause of the connection failure?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped.
The app server can ping the DB server, confirming Layer 3 connectivity, but telnet fails on port 3306, indicating a Layer 4 issue. Since the DB server's MySQL is listening on 0.0.0.0:3306 and firewalld allows the port, the most likely cause is that the router lacks a return route from the 10.0.0.0/24 subnet to the 192.168.1.0/24 subnet, causing return packets from the DB server to be dropped. This asymmetric routing prevents the TCP handshake from completing, even though ICMP (ping) may work if the router forwards it differently.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The network cable is faulty.
Why it's wrong here
If the cable were faulty, ping would also fail.
- ✗
The app server's firewall is blocking outgoing connections to port 3306.
- ✓
The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped.
Why this is correct
Even though ping works due to ICMP being handled differently, TCP connections require consistent routing. A missing return route would cause TCP SYNs to be sent but ACKs never received, resulting in connection timeout or refused.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The database service is only listening on the loopback interface (127.0.0.1).
Why it's wrong here
The stem says MySQL is listening on 0.0.0.0:3306, so this is false.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a successful ping implies full IP connectivity, but ICMP and TCP may be routed differently; the exam tests whether you understand that TCP requires symmetric routing for the handshake to complete, and a missing return route can cause a 'connection refused' even when the service is up and firewalls are open.
Detailed technical explanation
How to think about this question
When a TCP connection is initiated, the SYN packet reaches the DB server, but the SYN-ACK reply must be routed back to the app server's subnet. If the router does not have a route for 192.168.1.0/24 on its interface connected to 10.0.0.0/24, the return packet is dropped, causing the app server to see a 'connection refused' or timeout, depending on whether an ICMP unreachable is generated. This scenario is common in multi-subnet environments where static routes are misconfigured, and it highlights the importance of verifying bidirectional routing with tools like traceroute or checking the routing table on the router.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this LFCS question test?
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router does not have a route to 192.168.1.0/24 from the DB server's subnet, so return packets are dropped. — The app server can ping the DB server, confirming Layer 3 connectivity, but telnet fails on port 3306, indicating a Layer 4 issue. Since the DB server's MySQL is listening on 0.0.0.0:3306 and firewalld allows the port, the most likely cause is that the router lacks a return route from the 10.0.0.0/24 subnet to the 192.168.1.0/24 subnet, causing return packets from the DB server to be dropped. This asymmetric routing prevents the TCP handshake from completing, even though ICMP (ping) may work if the router forwards it differently.
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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