ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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
✓
ALB SG: inbound from 0.0.0.0/0 on port 443. Instance SG: inbound from ALB SG on port 443.
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.
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.
Why this is correct
Only allows HTTPS from ALB SG to instances.
- ✗
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.