This LPIC-1 practice question tests your understanding of essential system services and 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 sysadmin is troubleshooting a connectivity issue between two servers in different subnets. The output of 'traceroute 192.168.2.10' shows packets reaching a router but not the destination. The router's firewall uses iptables. Which rule would prevent the traceroute from completing?
-A INPUT -i lo -j LOG
Why wrong: Logs loopback traffic, does not drop.
-A INPUT -p udp -j ACCEPT
Why wrong: Accepts all UDP, would not block traceroute.
-A INPUT -p tcp --dport 80 -j REJECT
Why wrong: Affects TCP port 80, not traceroute (UDP).
-A INPUT -p icmp --icmp-type port-unreachable -j DROP
Blocks ICMP Port Unreachable, which stops traceroute responses.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-A INPUT -p icmp --icmp-type port-unreachable -j DROP
Option B is correct because traceroute sends UDP packets with increasing TTL, and the destination sends back ICMP Port Unreachable. Dropping ICMP would cause traceroute to hang. Option A is wrong because rejecting TCP is not relevant for traceroute (UDP). Option C is wrong because logging doesn't block. Option D is wrong because accepting all UDP would not block.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
-A INPUT -i lo -j LOG
Why it's wrong here
Logs loopback traffic, does not drop.
-A INPUT -p udp -j ACCEPT
Why it's wrong here
Accepts all UDP, would not block traceroute.
-A INPUT -p tcp --dport 80 -j REJECT
-A INPUT -p icmp --icmp-type port-unreachable -j DROP
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related LPIC-1 subnetting questions on CIDR, address ranges, and subnet selection.
Essential System Services and Networking — study guide chapter
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FAQ
Essential System Services and Networking — This question tests Essential System Services and Networking — CIDR notation defines the prefix length..
The correct answer is: -A INPUT -p icmp --icmp-type port-unreachable -j DROP — Option B is correct because traceroute sends UDP packets with increasing TTL, and the destination sends back ICMP Port Unreachable. Dropping ICMP would cause traceroute to hang. Option A is wrong because rejecting TCP is not relevant for traceroute (UDP). Option C is wrong because logging doesn't block. Option D is wrong because accepting all UDP would not block.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related LPIC-1 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
This LPIC-1 practice question is part of Courseiva's free LPI 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 LPIC-1 exam.
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