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.
Which THREE conditions must be met for a Linux system to function as a router between two networks?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Each interface has an IP address in the respective subnet
Option A is correct because each interface must have an IP address in its respective subnet for the Linux system to receive packets from that network and forward them to the other. Without an IP address in the subnet, the interface cannot participate in ARP resolution or routing decisions for that network.
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.
✓
Each interface has an IP address in the respective subnet
Why this is correct
The router must have an IP in each network to send/receive packets.
Related concept
Read the scenario before looking for a memorised answer.
✓
IP forwarding is enabled (net.ipv4.ip_forward = 1)
Why this is correct
This is required to forward packets between interfaces.
Related concept
Read the scenario before looking for a memorised answer.
✗
Both interfaces have the same MAC address
Why it's wrong here
Interfaces must have unique MAC addresses.
✓
iptables rules allow forwarding (FORWARD chain policy or rules)
Read the scenario before looking for a memorised answer.
✗
The system is configured as the default gateway for both networks
Why it's wrong here
The system can act as a router without being the default gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think a router must be the default gateway for both networks, but in reality, it only needs to have IP addresses in each subnet and proper routing entries; the default gateway is a client-side setting, not a router requirement.
Detailed technical explanation
How to think about this question
IP forwarding works at the kernel level by checking the routing table for a destination match; when net.ipv4.ip_forward is set to 1, the kernel will forward packets between interfaces if the destination IP is not local. The FORWARD chain in iptables must have a policy of ACCEPT or rules that permit traffic, as the default policy is often DROP for security. In a real-world scenario, a Linux router might use policy-based routing or NAT alongside forwarding.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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: Each interface has an IP address in the respective subnet — Option A is correct because each interface must have an IP address in its respective subnet for the Linux system to receive packets from that network and forward them to the other. Without an IP address in the subnet, the interface cannot participate in ARP resolution or routing decisions for that network.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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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.
Question Discussion
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