PCNE Configuring Network Services Practice Question
A company has deployed a web application on Compute Engine instances in a VPC with subnet 10.1.0.0/20. The instances need to access an external API that whitelists IP addresses. The company uses Cloud NAT to provide outbound connectivity. The API integration tests are failing, and the operations team suspects that the source IP addresses seen by the API are not consistent. What is the most likely cause and solution?
⚠ Common exam trap
Candidates often assume Cloud NAT automatically applies to all subnets in the VPC, but in reality, the default rule must explicitly include the subnet, and if it is removed or not configured, traffic from that subnet will not be NATed.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20.
If Cloud NAT's default rule does not include the subnet 10.1.0.0/20, instances in that subnet will not have their outbound traffic translated through the NAT gateway, causing them to use their ephemeral public IPs (if any) or fail to reach the external API. Adding a custom NAT rule that explicitly includes the subnet ensures all outbound traffic from those instances uses the consistent NAT IP address that the API whitelist expects.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Cloud NAT is configured with endpoint-independent mapping; change to endpoint-dependent mapping to ensure consistent source IP.
Why it's wrong here
Endpoint-independent mapping is the default and does not affect source IP consistency.
- ✗
Cloud NAT is configured with dynamic port allocation; use static port allocation instead.
Why it's wrong here
Dynamic port allocation affects port mapping, not source IP consistency.
- ✗
Cloud NAT is using a manual NAT IP address that is not assigned to the instances; assign the NAT IP to the instances as an alias IP range.
Why it's wrong here
NAT IPs are not assigned to instances; they are used by Cloud NAT for outbound traffic.
- ✓
Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20.
Why this is correct
If the subnet is not in a NAT rule, instances may not use NAT or use different NAT IPs, causing inconsistent source IPs. Adding the subnet ensures consistent NAT IP usage.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.