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.
A server has two network interfaces: eth0 (10.0.1.10/24, gateway 10.0.1.1) and eth1 (192.168.1.10/24, no gateway). Both are up. The default gateway is set to 10.0.1.1. A ping to 8.8.8.8 fails, but ping to 10.0.1.1 succeeds. What is the most likely cause?
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.
eth1 has no gateway configured
Why wrong: eth1 is on a private subnet and does not need a gateway for external access.
The default route is missing or pointing to an incorrect gateway
Even though the gateway is reachable, the default route may be missing or misconfigured.
DNS resolution is failing
Why wrong: Ping uses IP addresses, not DNS.
eth0 is down
Why wrong: If eth0 were down, ping to 10.0.1.1 would fail.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The default route is missing or pointing to an incorrect gateway
The default gateway is correctly set to 10.0.1.1, and ping to that gateway succeeds, so eth0 and its route are functional. However, ping to 8.8.8.8 fails, which indicates that the default route (0.0.0.0/0 via 10.0.1.1) is either missing or misconfigured, preventing traffic destined for external networks from being forwarded. The fact that eth1 has no gateway is irrelevant because the default route is already defined via eth0.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
eth1 has no gateway configured
Why it's wrong here
eth1 is on a private subnet and does not need a gateway for external access.
The default route is missing or pointing to an incorrect gateway
Why this is correct
Even though the gateway is reachable, the default route may be missing or misconfigured.
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.
DNS resolution is failing
Why it's wrong here
Ping uses IP addresses, not DNS.
eth0 is down
Why it's wrong here
If eth0 were down, ping to 10.0.1.1 would fail.
Common exam traps
The trap here is that candidates may incorrectly blame eth1's missing gateway (option A) instead of recognizing that the default route via eth0 is the sole path to external networks, and its absence is the real issue.
Detailed technical explanation
The default route (0.0.0.0/0) is the catch-all entry in the routing table that directs packets to destinations not matching any more specific route. If the default route is missing or points to an unreachable next-hop, the kernel will return 'Network is unreachable' or simply drop the packet. A common real-world scenario is when a network manager accidentally deletes the default route or a DHCP lease renewal fails to re-add it, causing all external connectivity to break while local LAN traffic continues to work.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: The default route is missing or pointing to an incorrect gateway — The default gateway is correctly set to 10.0.1.1, and ping to that gateway succeeds, so eth0 and its route are functional. However, ping to 8.8.8.8 fails, which indicates that the default route (0.0.0.0/0 via 10.0.1.1) is either missing or misconfigured, preventing traffic destined for external networks from being forwarded. The fact that eth1 has no gateway is irrelevant because the default route is already defined via eth0.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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