The answer is that the SSH rule restricted to the 10.0.0.0/8 subnet is the most likely cause, as the workstation’s IP of 192.168.1.100 falls outside that source range. In iptables, a rule that explicitly matches a source subnet will only permit traffic from addresses within that range; any packet from a different subnet, such as 192.168.0.0/16, will not match the rule and will be evaluated against the default policy. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of how iptables source matching works, often appearing as a trap where examinees overlook the subnet mask and assume all private IPs are allowed. A common memory tip is to think of iptables as a bouncer checking IDs: if the rule says “10.0.0.0/8 only,” a guest from 192.168.x.x is turned away at the door. Remember the mnemonic “SSH Subnet Snag” to always verify the source range before troubleshooting connectivity.
XK0-005 Security Practice Question
This XK0-005 practice question tests your understanding of 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.
Network Topology
A user on a workstation with IP 192.168.1.100 reports being unable to SSH to the server with IP 10.0.0.5. Based on the exhibit, 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 default INPUT policy is DROP and there is no SSH rule.
Why wrong: There is an SSH rule, but it is source-restricted.
B
The SSH rule appears after a LOG rule, causing it to be unreachable.
Why wrong: Rules are processed in order; the LOG rule does not affect acceptance of SSH because the LOG rule logs then continues to DROP, but the SSH rule is before the LOG and DROP rules.
C
The SSH rule only accepts connections from the 10.0.0.0/8 subnet.
The fifth line shows SSH accepted only from source 10.0.0.0/8; the user's IP is not in that range.
D
The SSH port is not allowed in any rule.
Why wrong: There is a rule accepting SSH on port 22, but only from 10.0.0.0/8.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SSH rule only accepts connections from the 10.0.0.0/8 subnet.
The exhibit shows an iptables ruleset where the SSH rule (port 22) explicitly matches the source address 10.0.0.0/8. The workstation has IP 192.168.1.100, which does not fall within the 10.0.0.0/8 range, so the SSH rule will not match. Since no other rule allows SSH, the packet will be evaluated against the default INPUT policy, which is ACCEPT (not shown as DROP), but the SSH rule itself restricts the source, making it unreachable for this client.
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.
✗
The default INPUT policy is DROP and there is no SSH rule.
Why it's wrong here
There is an SSH rule, but it is source-restricted.
✗
The SSH rule appears after a LOG rule, causing it to be unreachable.
Why it's wrong here
Rules are processed in order; the LOG rule does not affect acceptance of SSH because the LOG rule logs then continues to DROP, but the SSH rule is before the LOG and DROP rules.
✓
The SSH rule only accepts connections from the 10.0.0.0/8 subnet.
Why this is correct
The fifth line shows SSH accepted only from source 10.0.0.0/8; the user's IP is not in that range.
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 SSH port is not allowed in any rule.
Why it's wrong here
There is a rule accepting SSH on port 22, but only from 10.0.0.0/8.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume any rule with 'ACCEPT' for SSH is sufficient, overlooking the source address restriction, and they may also mistakenly think a LOG rule blocks subsequent rules, when in fact LOG is non-terminating.
Detailed technical explanation
How to think about this question
In iptables, rules are evaluated in order, and a LOG rule uses the LOG target which does not change the packet's fate—it simply logs and continues to the next rule. The SSH rule uses the ACCEPT target, which is terminating, but only if the source address matches 10.0.0.0/8. This is a common misconfiguration when administrators intend to allow SSH from a specific subnet but inadvertently use a too-broad or incorrect subnet mask, or forget to include the management subnet. In real-world scenarios, such a mistake can lock out remote administrators if the management interface is on a different subnet.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The SSH rule only accepts connections from the 10.0.0.0/8 subnet. — The exhibit shows an iptables ruleset where the SSH rule (port 22) explicitly matches the source address 10.0.0.0/8. The workstation has IP 192.168.1.100, which does not fall within the 10.0.0.0/8 range, so the SSH rule will not match. Since no other rule allows SSH, the packet will be evaluated against the default INPUT policy, which is ACCEPT (not shown as DROP), but the SSH rule itself restricts the source, making it unreachable for this client.
What should I do if I get this XK0-005 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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