- A
The ALB's public IP address
Why wrong: ALB public IPs can change, and using IPs is less flexible.
- B
The subnet CIDR block of the ALB's subnets
Why wrong: This allows traffic from all instances in those subnets, not just the ALB.
- C
The ID of the ALB's security group
Referencing the ALB's security group as a source ensures only traffic from the ALB is allowed.
- D
The VPC CIDR block
Why wrong: This allows traffic from any resource in the VPC, not just the ALB.
Question 900 of 1,705
Using Security Group Referencing to Allow Traffic from ALB to Targets
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 an Application Load Balancer (ALB) in front of a fleet of EC2 instances. The security group for the EC2 instances must allow traffic only from the ALB. Which source should be specified in the security group inbound rule?
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
✓
The ID of the ALB's security group
Option C is correct because you can reference the ALB's security group ID as the source in the EC2 instances' security group inbound rule. This allows traffic from any network interface that is associated with the ALB's security group, regardless of the ALB's IP addresses or subnets. This is the recommended AWS best practice for controlling traffic between an ALB and its target instances, as it automatically adapts to changes in the ALB's underlying infrastructure.
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.
- ✗
The ALB's public IP address
Why it's wrong here
ALB public IPs can change, and using IPs is less flexible.
- ✗
The subnet CIDR block of the ALB's subnets
Why it's wrong here
This allows traffic from all instances in those subnets, not just the ALB.
- ✓
The ID of the ALB's security group
Why this is correct
Referencing the ALB's security group as a source ensures only traffic from the ALB is allowed.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The VPC CIDR block
Why it's wrong here
This allows traffic from any resource in the VPC, not just the ALB.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume they need to use the ALB's IP addresses or subnet CIDR, not realizing that AWS security groups can reference other security groups as a source, which is the most secure and scalable method for this scenario.
Detailed technical explanation
How to think about this question
When you reference a security group ID as a source in an inbound rule, AWS evaluates the rule based on the network interfaces (ENIs) that are members of that security group. For an ALB, each of its nodes has an ENI in the ALB's subnets, and those ENIs are associated with the ALB's security group. This mechanism works even if the ALB is internet-facing and its public IPs change, because the security group membership is tied to the ENI, not the IP address. This approach also simplifies cross-account or cross-VPC ALB-to-target traffic when using VPC peering or AWS PrivateLink.
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.
Visual reference
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ID of the ALB's security group — Option C is correct because you can reference the ALB's security group ID as the source in the EC2 instances' security group inbound rule. This allows traffic from any network interface that is associated with the ALB's security group, regardless of the ALB's IP addresses or subnets. This is the recommended AWS best practice for controlling traffic between an ALB and its target instances, as it automatically adapts to changes in the ALB's underlying infrastructure.
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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Same concept, more angles
1 more ways this is tested on ANS-C01
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 has a VPC with an application load balancer (ALB) in public subnets and EC2 instances in private subnets. The EC2 instances must only accept traffic from the ALB. Which security group configuration achieves this?
easy- A.Allow inbound traffic from 0.0.0.0/0 on the application port.
- B.Allow inbound traffic from the VPC CIDR on the application port.
- C.Allow inbound traffic from the private subnet CIDR on the application port.
- ✓ D.Allow inbound traffic from the ALB's security group on the application port.
Why D: Option D is correct because referencing the ALB's security group as the source in the EC2 instances' inbound rule ensures that only traffic originating from the ALB is allowed. This leverages AWS security group referencing, which dynamically resolves to the private IP addresses of the ALB's elastic network interfaces (ENIs), providing a secure and scalable solution without exposing the instances to broader network ranges.
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Last reviewed: Jul 4, 2026
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.
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