The answer is to configure the ALB security group with an inbound rule allowing HTTPS traffic on port 443 from 0.0.0.0/0, and the backend EC2 instance security group with an inbound rule allowing port 443 traffic only from the ALB security group. This is correct because it enforces the principle of least privilege by creating a security group reference, which ensures that only the ALB’s elastic network interfaces can initiate traffic to the instances, preventing any direct internet or cross-subnet access to the private EC2 fleet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateful security group chaining versus network ACLs, and a common trap is to mistakenly allow the instance security group to accept traffic from the ALB’s CIDR block or from 0.0.0.0/0, which would expose the backend. A reliable memory tip is to think of the security group as a “VIP pass” — the ALB holds the pass, and the instance only lets in traffic that shows that pass, not just any traffic from a specific IP range.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 public and private subnets in three Availability Zones. They have an Application Load Balancer (ALB) in the public subnets and a fleet of EC2 instances in the private subnets. The ALB needs to send traffic to the instances on port 443. What is the most secure way to configure the security groups?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from ALB SG on port 443.
Option C is correct because it follows the principle of least privilege by restricting the ALB security group to only allow inbound traffic on port 443 from the internet (0.0.0.0/0) and the instance security group to only allow inbound traffic on port 443 from the ALB security group. This ensures that only the ALB can communicate with the instances on the required port, preventing direct access from other sources and reducing the attack surface.
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.
✗
ALB SG: inbound from 0.0.0.0/0 on all ports. Instance SG: inbound from ALB SG on all ports.
Why it's wrong here
ALB SG allows all ports from internet, which is insecure.
✗
ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from 0.0.0.0/0 on port 443.
Why it's wrong here
Instances are exposed to internet.
✓
ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from ALB SG on port 443.
Read the scenario before looking for a memorised answer.
✗
ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from ALB SG on all ports.
Why it's wrong here
Instance SG allows all ports, which is not least privilege.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume that allowing traffic from 0.0.0.0/0 to the instances is acceptable because the instances are in private subnets, but security groups are stateful and do not consider subnet routing; they evaluate traffic based on the source IP, so a rule allowing 0.0.0.0/0 would permit direct internet traffic if the instances had a route to an internet gateway.
Detailed technical explanation
How to think about this question
When using security group references as a source, AWS evaluates the rule based on the private IP addresses of the instances associated with the referenced security group, not the public IPs. This allows the ALB to communicate with the instances even if the instances are in private subnets without public IPs. In a real-world scenario, this configuration is critical for maintaining a secure web application architecture where the ALB terminates TLS and forwards traffic to backend instances, ensuring that the instances are not directly accessible from the internet.
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.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from ALB SG on port 443. — Option C is correct because it follows the principle of least privilege by restricting the ALB security group to only allow inbound traffic on port 443 from the internet (0.0.0.0/0) and the instance security group to only allow inbound traffic on port 443 from the ALB security group. This ensures that only the ALB can communicate with the instances on the required port, preventing direct access from other sources and reducing the attack surface.
What should I do if I get this ANS-C01 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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