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.
Which TWO iptables commands are necessary to allow incoming SSH connections (port 22) from the subnet 192.168.1.0/24 while dropping all other incoming traffic? Assume the INPUT chain has a default policy of DROP. (Choose two.)
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 -j DROP
Options A and D are correct. Option A adds a rule to accept SSH from the subnet. Option D adds a rule to drop all other (or it might be unnecessary if default policy is DROP, but to be explicit, it's often added). Option B is wrong because it accepts all traffic from the subnet, not just SSH. Option C is wrong because it drops all traffic from the subnet. Option E is wrong because it accepts all incoming traffic.
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 -s 192.168.1.0/24 -p tcp --dport 22 -j DROP
Why it's wrong here
This drops SSH from the subnet, opposite of desired.
✓
iptables -A INPUT -j DROP
Why this is correct
This drops all remaining incoming traffic not matched by earlier rules.
This would accept all traffic, defeating the purpose of dropping other traffic.
✗
iptables -A INPUT -s 192.168.1.0/24 -j ACCEPT
Why it's wrong here
This allows all traffic from the subnet, not just SSH.
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.
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 -j DROP — Options A and D are correct. Option A adds a rule to accept SSH from the subnet. Option D adds a rule to drop all other (or it might be unnecessary if default policy is DROP, but to be explicit, it's often added). Option B is wrong because it accepts all traffic from the subnet, not just SSH. Option C is wrong because it drops all traffic from the subnet. Option E is wrong because it accepts all incoming traffic.
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.
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Question Discussion
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