Quick Answer
The answer is that the hosts on 10.0.1.0/24 lack a route to the 10.0.2.0/24 network. This is the most likely cause because when a host tries to ping across subnets, its kernel checks its local routing table for a path to the destination; if no route exists for 10.0.2.0/24, the packet is dropped at the source before it ever reaches the router. The Linux router itself can ping both subnets, confirming that IP forwarding is enabled and the iptables FORWARD chain is not blocking traffic, which isolates the problem to the sending host’s missing route. On the LFCS exam, this scenario tests your understanding that a router’s ability to forward packets does not compensate for a client’s lack of a default gateway or static route to the remote subnet—a common trap where candidates mistakenly blame firewall rules or forwarding settings. Remember the mnemonic: “The router can roam, but the host needs a home—without a route, the packet never leaves the dome.”
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 troubleshooting a Linux server that acts as a router between two networks: 10.0.1.0/24 (eth0) and 10.0.2.0/24 (eth1). IP forwarding is enabled. Hosts on 10.0.1.0/24 can ping the server's eth0 IP (10.0.1.1), but cannot ping hosts on 10.0.2.0/24 (e.g., 10.0.2.10). The server can ping both 10.0.2.10 and 10.0.1.10. The iptables FORWARD chain policy is ACCEPT. 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.
The hosts on 10.0.1.0/24 do not have a route to 10.0.2.0/24
If hosts don't know where 10.0.2.0/24 is, they send packets to their default gateway, which may not be this server.
The server cannot reach 10.0.2.10
Why wrong: The server can ping 10.0.2.10.
IP forwarding is not enabled
Why wrong: The scenario states IP forwarding is enabled.
The iptables FORWARD chain has a DROP policy
Why wrong: The policy is ACCEPT.
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 hosts on 10.0.1.0/24 do not have a route to 10.0.2.0/24
The server can ping both networks, confirming IP forwarding is enabled and iptables FORWARD chain policy is ACCEPT. Hosts on 10.0.1.0/24 can ping the server's eth0 IP but not hosts on 10.0.2.0/24, which indicates the hosts lack a route to the 10.0.2.0/24 network. Without a route, packets from 10.0.1.0/24 destined for 10.0.2.0/24 are dropped at the source host because the kernel has no next-hop information for that destination.
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.
The hosts on 10.0.1.0/24 do not have a route to 10.0.2.0/24
Why this is correct
If hosts don't know where 10.0.2.0/24 is, they send packets to their default gateway, which may not be this server.
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 server cannot reach 10.0.2.10
Why it's wrong here
The server can ping 10.0.2.10.
IP forwarding is not enabled
Why it's wrong here
The scenario states IP forwarding is enabled.
The iptables FORWARD chain has a DROP policy
Why it's wrong here
The policy is ACCEPT.
Common exam traps
The trap here is that candidates often assume the router's IP forwarding or firewall is the problem when the server itself can reach both networks, overlooking the requirement for client-side routing configuration.
Trap categories for this question
Scenario analysis trap
The scenario states IP forwarding is enabled.
Detailed technical explanation
For a Linux router to forward traffic between subnets, each host must have a route to the remote network via the router's IP as the gateway. This is typically configured with the `ip route add` command or via DHCP option 3 (router). Without this route, the host's kernel performs a routing table lookup, finds no match, and returns an ICMP Destination Unreachable (Network Unreachable) to the application, even though the router is capable of forwarding.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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 hosts on 10.0.1.0/24 do not have a route to 10.0.2.0/24 — The server can ping both networks, confirming IP forwarding is enabled and iptables FORWARD chain policy is ACCEPT. Hosts on 10.0.1.0/24 can ping the server's eth0 IP but not hosts on 10.0.2.0/24, which indicates the hosts lack a route to the 10.0.2.0/24 network. Without a route, packets from 10.0.1.0/24 destined for 10.0.2.0/24 are dropped at the source host because the kernel has no next-hop information for that destination.
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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