- A
iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Allows SSH from subnet, then drops all other input.
- B
iptables -A INPUT -p tcp --sport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Why wrong: --sport specifies source port, which is not the port we are connecting to (SSH uses destination port 22).
- C
iptables -A INPUT -p tcp --dport 22 -j DROP iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 22 -j ACCEPT
Why wrong: DROP rule before ACCEPT rule will drop all SSH, then the second rule is never evaluated.
- D
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j DROP
Why wrong: First rule allows all SSH, then second rule does not override; packets from subnet will still be allowed.
- E
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -j DROP
Why wrong: Allows SSH from any source, then drops all other input; does not restrict SSH to subnet.
LPIC-2 System Security Practice Question
This LPIC-2 practice question tests your understanding of system security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 firewall rule set is implemented using iptables. The administrator wants to allow incoming SSH connections only from the 192.168.1.0/24 subnet, while all other incoming traffic is dropped. Which set of rules achieves this?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Option D is correct. First, allow SSH from the subnet, then drop all other incoming traffic. Option A drops before allowing, B uses wrong syntax (dport with --sport), C allows all SSH before restricting, and E is correct but uses DROP instead of REJECT; however, the question does not specify preference, but D is more standard. Actually, D uses DROP which is fine. Option E allows SSH from anywhere then restricts? No, E uses both ACCEPT and DROP in order; first rule allows all SSH, second drops all input, so SSH from anywhere would be allowed. So D is best.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
- ✗
iptables -A INPUT -p tcp --sport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP
Why it's wrong here
--sport specifies source port, which is not the port we are connecting to (SSH uses destination port 22).
- ✗
iptables -A INPUT -p tcp --dport 22 -j DROP iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 22 -j ACCEPT
Why it's wrong here
DROP rule before ACCEPT rule will drop all SSH, then the second rule is never evaluated.
- ✗
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j DROP
Why it's wrong here
First rule allows all SSH, then second rule does not override; packets from subnet will still be allowed.
- ✗
iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -j DROP
Why it's wrong here
Allows SSH from any source, then drops all other input; does not restrict SSH to subnet.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
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
How to think about this question
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.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
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
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
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-2 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this LPIC-2 question test?
System Security — This question tests System Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -j DROP — Option D is correct. First, allow SSH from the subnet, then drop all other incoming traffic. Option A drops before allowing, B uses wrong syntax (dport with --sport), C allows all SSH before restricting, and E is correct but uses DROP instead of REJECT; however, the question does not specify preference, but D is more standard. Actually, D uses DROP which is fine. Option E allows SSH from anywhere then restricts? No, E uses both ACCEPT and DROP in order; first rule allows all SSH, second drops all input, so SSH from anywhere would be allowed. So D is best.
What should I do if I get this LPIC-2 question wrong?
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-2 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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Last reviewed: Jun 24, 2026
This LPIC-2 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-2 exam.
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