The simplest firewall rule to allow HTTP from the internet using implicit deny is to create a single ingress rule that permits TCP port 80 from 0.0.0.0/0 and apply it to instances tagged as 'web-server'. This works because cloud VPC firewalls, including those in Google Cloud, operate on an implicit deny principle—all inbound traffic is blocked by default unless an explicit allow rule exists. By adding only the HTTP allow rule, you satisfy the requirement to permit web traffic from the internet while the implicit deny automatically blocks all other inbound traffic, no additional rules needed. On the Google Professional Cloud Security Engineer exam, this tests your understanding of VPC firewall evaluation logic and the default deny stance, a common trap being the mistaken belief you need a separate deny rule. Remember the memory tip: "One allow, implicit deny does the rest"—you only write rules for what you want to permit, and the platform handles the rest.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 has a VPC with several subnets. They want to allow HTTP traffic from the internet to a web server in subnet-a, but block all other inbound traffic. What is the simplest firewall rule configuration?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create an ingress rule allowing tcp:80 from 0.0.0.0/0 to instances with the 'web-server' tag. The implied deny all ingress rule blocks other traffic.
Option C is correct because it creates an ingress rule that explicitly allows TCP port 80 traffic from any source (0.0.0.0/0) to instances tagged as 'web-server'. In cloud VPC firewalls (e.g., AWS Security Groups or GCP Firewall Rules), the default behavior is an implied deny all ingress traffic; only explicitly allowed traffic is permitted. This configuration satisfies the requirement to allow HTTP from the internet while blocking all other inbound traffic without needing additional rules.
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.
✗
Create an ingress rule allowing all tcp and udp traffic from 0.0.0.0/0.
Why it's wrong here
Incorrect: too permissive.
✗
Create an egress rule allowing tcp:80 from the internet to the web server.
Why it's wrong here
Incorrect: egress rules control outbound, not inbound.
✓
Create an ingress rule allowing tcp:80 from 0.0.0.0/0 to instances with the 'web-server' tag. The implied deny all ingress rule blocks other traffic.
Why this is correct
Correct: single rule with tag and source range, plus implied deny.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create an ingress rule allowing tcp:80 from the web server's internal IP range. The implied deny all ingress rule blocks other traffic.
Why it's wrong here
Incorrect: source should be 0.0.0.0/0 for internet traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between ingress and egress rules, and the trap here is that candidates mistakenly think an egress rule (Option B) can control inbound traffic, or that restricting to internal IPs (Option D) is sufficient for internet access.
Detailed technical explanation
How to think about this question
In cloud VPC firewalls, rules are stateful: if an ingress rule allows HTTP traffic, the corresponding return traffic is automatically permitted, so no separate egress rule is needed. The implied deny all ingress rule is a fundamental security principle that ensures only explicitly allowed traffic enters the subnet; this is implemented as a default rule with lowest priority that drops all traffic not matching a higher-priority allow rule. Tag-based rules (e.g., 'web-server') allow scalable management by applying the rule to any instance with that tag, rather than hardcoding IP addresses.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 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: Create an ingress rule allowing tcp:80 from 0.0.0.0/0 to instances with the 'web-server' tag. The implied deny all ingress rule blocks other traffic. — Option C is correct because it creates an ingress rule that explicitly allows TCP port 80 traffic from any source (0.0.0.0/0) to instances tagged as 'web-server'. In cloud VPC firewalls (e.g., AWS Security Groups or GCP Firewall Rules), the default behavior is an implied deny all ingress traffic; only explicitly allowed traffic is permitted. This configuration satisfies the requirement to allow HTTP from the internet while blocking all other inbound traffic without needing additional rules.
What should I do if I get this PCSE 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.
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Question Discussion
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