The answer is an incorrect subnet mask on eth0, which causes the gateway to be considered on a different network. When you cannot ping the default gateway and get destination host unreachable in Linux, the kernel first checks whether the target IP falls within the same subnet as the local interface; if the subnet mask is wrong, the gateway’s address appears to be on a separate network, so the system never sends an ARP request and instead immediately returns the unreachable error. This scenario tests your understanding of how the Linux routing table and interface configuration interact, a common CompTIA Linux+ XK0-005 exam trap where candidates mistakenly blame the default route or the gateway itself. Remember that a working loopback and local IP ping confirm the stack is fine, so the issue must be at Layer 3—specifically, the subnet mask misaligning the gateway’s network. A useful memory tip: “If the gateway seems foreign, check the mask—it’s the border guard.”
XK0-005 Troubleshooting Practice Question
This XK0-005 practice question tests your understanding of 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 Linux administrator is troubleshooting network connectivity. The server can ping its own IP address but cannot ping the default gateway. The output of 'ip route show' is: 'default via 10.0.0.1 dev eth0 proto static metric 100'. The output of 'ping -c 1 10.0.0.1' fails with 'Destination Host Unreachable'. Which of the following 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The subnet mask on eth0 is incorrect, causing the gateway to be considered on a different network.
The server can ping its own IP address, confirming that the local network stack and the eth0 interface are operational. However, the 'Destination Host Unreachable' error when pinging the default gateway (10.0.0.1) indicates that the host does not have a valid route to that destination. Since the default route exists, the most likely cause is an incorrect subnet mask on eth0, which causes the kernel to treat the gateway as being on a different network, thus failing to send ARP requests or forward packets to it.
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 eth0 interface is down.
Why it's wrong here
If eth0 were down, the own IP ping would also fail.
✗
The gateway is down or not responding.
Why it's wrong here
A down gateway would typically cause timeouts, not 'unreachable' from the local host.
The subnet mask on eth0 is incorrect, causing the gateway to be considered on a different network.
Why this is correct
A wrong subnet mask can make the gateway appear on a different subnet, leading to 'unreachable'.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume 'Destination Host Unreachable' always means the gateway is down, but in Linux this error specifically indicates the local host cannot find a layer-2 path to the destination, typically due to a subnet mask mismatch or missing ARP entry.
Trap categories for this question
Command / output trap
The route table shows a default gateway is set.
Detailed technical explanation
How to think about this question
When a host sends a packet to a destination IP, the kernel performs a longest-prefix-match lookup in the routing table. If the destination is not on a directly connected subnet (as determined by the interface IP and subnet mask), the packet is forwarded to the default gateway. If the subnet mask on eth0 is incorrect (e.g., /24 instead of /16), the kernel may calculate that 10.0.0.1 is not within the local subnet, causing it to attempt to route the packet via the default gateway itself—creating a loop or a 'Destination Host Unreachable' because no ARP entry can be created for a non-local address. This is a common misconfiguration in static IP setups where the subnet mask does not match the actual network topology.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The subnet mask on eth0 is incorrect, causing the gateway to be considered on a different network. — The server can ping its own IP address, confirming that the local network stack and the eth0 interface are operational. However, the 'Destination Host Unreachable' error when pinging the default gateway (10.0.0.1) indicates that the host does not have a valid route to that destination. Since the default route exists, the most likely cause is an incorrect subnet mask on eth0, which causes the kernel to treat the gateway as being on a different network, thus failing to send ARP requests or forward packets to it.
What should I do if I get this XK0-005 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A Linux server with the IP address 192.168.1.100 is unable to communicate with other hosts on the same subnet 192.168.1.0/24. The administrator can ping the loopback address, but pinging 192.168.1.1 (the default gateway) fails. The output of `ip a` shows the eth0 interface has the correct IP and netmask. Which troubleshooting step should be performed next?
easy
A.Check the ARP cache with arp -a.
B.Replace the network cable.
C.Restart the network service.
✓ D.Check the routing table with ip route.
Why D: Since the server has the correct IP and netmask on eth0 but cannot ping the default gateway (192.168.1.1), the issue likely lies in the routing configuration. The `ip route` command displays the kernel routing table, including the default gateway entry; if the default route is missing or incorrect, traffic cannot reach the gateway. Checking the routing table is the logical next step before assuming physical or ARP-level problems.
Last reviewed: Jun 11, 2026
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This XK0-005 practice question is part of Courseiva's free CompTIA 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 XK0-005 exam.
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