The answer is to verify that the external service is not blocking the NAT IP addresses and to check Cloud NAT logs for port exhaustion. When troubleshooting Cloud NAT connectivity issues, port exhaustion occurs because all private instances in a VPC share a finite pool of source ports on the same NAT IP addresses; if some instances generate a high volume of outbound connections, they can consume all available ports, starving other instances and causing them to fail while others succeed. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of Cloud NAT’s port allocation behavior and the common trap of assuming misconfigured tags or subnets are the cause, when the real issue is resource contention at the NAT gateway. A reliable memory tip is “ports before IPs”—always check for port exhaustion first when connectivity is intermittent, then verify that the destination isn’t blacklisting your NAT IPs.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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 company uses Cloud NAT to enable outbound internet access for private instances in a VPC. They notice that some instances are unable to connect to external services, while others can. The network team has verified that all instances have the same tags and are in the same subnet. Which TWO actions should the team take to troubleshoot the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check Cloud NAT logs for port exhaustion
Option B is correct because Cloud NAT logs can reveal port exhaustion, which occurs when the NAT gateway runs out of available source ports for connections. Since all instances share the same NAT IP addresses, high connection counts from some instances can exhaust ports, preventing others from establishing new outbound connections. Checking these logs helps identify whether port exhaustion is the root cause of the connectivity issue.
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.
✗
Ensure Cloud NAT is enabled in each zone where instances are located
Why it's wrong here
Cloud NAT is regional; it covers all zones in the region.
✓
Check Cloud NAT logs for port exhaustion
Why this is correct
Port exhaustion can cause connection failures; Cloud NAT logs show when ports are depleted.
Related concept
Read the scenario before looking for a memorised answer.
✗
Verify that the instances have external IP addresses assigned
Why it's wrong here
Cloud NAT works with instances that have only internal IPs; external IPs would bypass NAT.
✓
Verify that the external service is not blocking the NAT IP addresses
Why this is correct
External services may block or rate-limit requests from certain IPs, causing failures.
Related concept
Read the scenario before looking for a memorised answer.
✗
Review the health check configuration for the NAT gateway
Why it's wrong here
Cloud NAT does not have health checks; it routes traffic directly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that Cloud NAT must be zone-specific or that instances need external IPs for NAT to work, but the real trap is overlooking port exhaustion as a common failure mode when multiple instances share the same NAT gateway.
Detailed technical explanation
How to think about this question
Cloud NAT uses source network address translation (SNAT) to map private instance IPs to a pool of public IP addresses and ephemeral source ports. Each connection consumes a unique source port (e.g., TCP/UDP port), and the total available ports per NAT IP is approximately 64,512. When many concurrent connections are made, especially from a few instances, port exhaustion can occur, causing new connections to fail. Cloud NAT logs (enabled via VPC flow logs or NAT gateway logging) capture connection attempts and can show 'port allocation failed' errors, pinpointing the issue.
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 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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check Cloud NAT logs for port exhaustion — Option B is correct because Cloud NAT logs can reveal port exhaustion, which occurs when the NAT gateway runs out of available source ports for connections. Since all instances share the same NAT IP addresses, high connection counts from some instances can exhaust ports, preventing others from establishing new outbound connections. Checking these logs helps identify whether port exhaustion is the root cause of the connectivity issue.
What should I do if I get this PCNE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is using Cloud NAT to allow private instances to access the internet. They notice that some instances are not able to reach certain external services. Which THREE steps should they take to troubleshoot?
hard
✓ A.Check if the external service is blocking the Cloud NAT IP addresses.
✓ B.Verify that the VPC firewall rules allow egress traffic from the instances.
C.Ensure that the VPC has a default route to the internet gateway.
✓ D.Verify that the Cloud Router associated with Cloud NAT is healthy and has established BGP sessions.
E.Check the DNS resolution for the external service.
Why A: Option A is correct because Cloud NAT uses a set of public IP addresses for outbound traffic. If the external service has firewall rules or access control lists that block these specific IP addresses, the instances will be unable to reach it. Checking for IP-based blocking is a standard first step in troubleshooting connectivity issues through a NAT gateway.
Variation 2. A company uses Cloud NAT to allow private VMs to access the internet. They notice that some VMs are unable to reach a specific set of external IP addresses, but other VMs can. The firewall rules are correctly configured. What is the most likely cause?
hard
A.The default route (0.0.0.0/0) is missing for the VPC.
B.The VPC firewall rules have a deny rule for the source IP range of the affected VMs.
✓ C.The Cloud NAT gateway uses a static IP address that is not allowed by the external service's firewall.
D.The VMs have a tag that overrides the Cloud NAT routing.
Why C: Cloud NAT uses a source IP address (either a static IP you assign or an ephemeral IP from the NAT gateway's IP range) when translating outbound traffic from private VMs. If the external service's firewall only allows traffic from specific IP addresses, and the Cloud NAT gateway is using a static IP that is not on that allowlist, the affected VMs' traffic will be blocked. Other VMs might reach the service if they use a different NAT gateway or if the service's firewall permits their translated IP.
Variation 3. A company uses Cloud NAT for outbound internet access. Engineering notices that some VM instances fail to connect to external services during peak hours. The network engineer suspects port exhaustion. Which action would best mitigate this issue?
medium
A.Change the NAT type from standard to public.
B.Enable TCP keepalive on the NAT gateway.
C.Reduce the number of NAT gateways in the region.
✓ D.Increase the number of NAT IP addresses assigned to the NAT gateway.
Why D: Port exhaustion occurs when the NAT gateway runs out of available source ports to map outbound connections. Increasing the number of NAT IP addresses assigned to the gateway multiplies the available port range (each IP provides 64,512 ports), directly alleviating port exhaustion during peak traffic. This is the standard mitigation for port exhaustion in Cloud NAT.
Last reviewed: Jun 30, 2026
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This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.
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