The answer is that the ephemeral external IP changed after a stop/start cycle, so users are likely trying to reach an old address. This is the most likely cause because an ephemeral external IP is not permanently assigned to a Compute Engine instance; it is released when the instance stops and a new one is assigned upon restart, breaking connectivity for anyone using the previous IP. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of the difference between ephemeral and static external IPs, and it often appears as a distractor where examinees overthink firewall rules or routing. The common trap is to assume the issue is with VPC firewall rules or iptables, but since the instance has the correct tag and the OS firewall is default allow, the real culprit is the IP volatility. Memory tip: "Ephemeral equals temporary—if it stops, the IP drops."
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 small company has a single VPC with one subnet in us-central1 (10.0.1.0/24). They have a Compute Engine instance that needs to be reachable from the internet via HTTP (port 80) and HTTPS (port 443). The instance has an external IP address (ephemeral). They have created firewall rules allowing ingress on TCP 80 and 443 from 0.0.0.0/0, with target tags 'web-server'. The instance has been assigned the tag 'web-server'. However, external users report that they cannot access the instance's public IP on either port. The instance's OS firewall (iptables) is default allow. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance's external IP is ephemeral and may have changed, so users should use the new IP or reserve a static IP.
The instance likely has an ephemeral external IP that may have changed after a stop/start, or the firewall rules are not properly applied. Since they have a tag, the rule should work. But a common mistake is not having a default route (0.0.0.0/0) in the VPC to allow internet traffic to reach the instance. Actually, the VPC automatically has a default route that sends traffic to the internet gateway, so that should be fine. Another possibility is that the instance's firewall (iptables) is blocking. But the most likely cause is that the external IP is ephemeral and may have changed, and users are using the old IP. Option B is correct: The external IP might have changed after a restart.
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.
✓
The instance's external IP is ephemeral and may have changed, so users should use the new IP or reserve a static IP.
Why this is correct
Ephemeral IPs can change, causing connectivity issues; a static IP is recommended.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The firewall rules must be applied to the subnet, not the instance tag.
Why it's wrong here
Using tags is a valid method to apply firewall rules.
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 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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instance's external IP is ephemeral and may have changed, so users should use the new IP or reserve a static IP. — The instance likely has an ephemeral external IP that may have changed after a stop/start, or the firewall rules are not properly applied. Since they have a tag, the rule should work. But a common mistake is not having a default route (0.0.0.0/0) in the VPC to allow internet traffic to reach the instance. Actually, the VPC automatically has a default route that sends traffic to the internet gateway, so that should be fine. Another possibility is that the instance's firewall (iptables) is blocking. But the most likely cause is that the external IP is ephemeral and may have changed, and users are using the old IP. Option B is correct: The external IP might have changed after a restart.
What should I do if I get this PCNE 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 PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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