The answer is that the user’s IP address 198.51.100.1 falls outside the allowed source range defined in the firewall rule. SSH access is denied because the configured VPC firewall rule permits incoming SSH traffic only from the source IP range 203.113.0.0/24, and the user’s IP does not match that CIDR block. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of hierarchical firewall rules and how source IP filtering works at the VPC level—a common trap is assuming that instance tags alone grant access, when in fact the source range must also be satisfied. Remember that firewall rules are evaluated in order of priority, and if no rule matches the source IP, the default implicit deny blocks the connection. A useful memory tip: “Tag opens the door, but source range decides who walks through.”
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 user is unable to SSH into an instance that has the tag 'ssh-access' and an internal IP 10.0.0.2. The user's IP is 198.51.100.1. What is the most likely reason?
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 instance is not using the correct service account
Why wrong: Service accounts are not relevant for network connectivity; they control API access.
B
The instance does not have an external IP
Why wrong: The instance has an internal IP, but SSH can be done via internal IP if within VPC or via VPN. The rule allows from a specific source range, not based on external IP.
C
The user's IP is not in the allowed source range
The rule's sourceRanges only includes 203.0.113.0/24, not the user's IP.
D
The firewall rule is disabled
Why wrong: The rule is described, and there is no indication it is disabled.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The user's IP is not in the allowed source range
The firewall rule only allows SSH from the source range 203.0.113.0/24. The user's IP (198.51.100.1) is not in that range, so the connection is denied.
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 instance is not using the correct service account
Why it's wrong here
Service accounts are not relevant for network connectivity; they control API access.
✗
The instance does not have an external IP
Why it's wrong here
The instance has an internal IP, but SSH can be done via internal IP if within VPC or via VPN. The rule allows from a specific source range, not based on external IP.
✓
The user's IP is not in the allowed source range
Why this is correct
The rule's sourceRanges only includes 203.0.113.0/24, not the user's IP.
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 firewall rule is disabled
Why it's wrong here
The rule is described, and there is no indication it is disabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which PCSE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The user's IP is not in the allowed source range — The firewall rule only allows SSH from the source range 203.0.113.0/24. The user's IP (198.51.100.1) is not in that range, so the connection is denied.
What should I do if I get this PCSE question wrong?
Identify which PCSE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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