- A
Configure the Application Load Balancer's security group to allow inbound from corporate IPs, and the EC2 instances' security group to allow inbound from the ALB's security group.
This ensures that only traffic through the ALB (including health checks) reaches the instances, and the ALB only accepts corporate IP traffic.
- B
Create two target groups: one for corporate traffic and one for health check traffic, each with different security groups.
Why wrong: Health checks are sent by the ALB itself, not by separate target groups; this approach doesn't solve the problem.
- C
Use AWS WAF on the ALB to create an IP set containing corporate IPs, and allow all traffic from ALB to instances.
Why wrong: AWS WAF can filter by IP, but health checker traffic from the ALB is still allowed; however, WAF adds unnecessary cost and complexity compared to security groups.
- D
Use a network ACL on the VPC subnet to allow inbound traffic from corporate IPs and AWS health checker IP ranges.
Why wrong: NACLs are stateless and would require explicitly allowing health checker traffic, but health checker IPs are not fixed; using security groups is simpler.
Quick Answer
The correct answer is to configure the Application Load Balancer’s security group to allow inbound traffic from the corporate IPs, and the EC2 instances’ security group to allow inbound traffic only from the ALB’s security group. This works because AWS health checkers originate from the ALB’s private IP addresses, so allowing the ALB security group as the source for the instance security group automatically permits health check traffic without exposing the instances to the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of security group chaining versus stateless NACLs, which would require cumbersome return traffic rules and are not the recommended solution. A common trap is to overcomplicate the design with WAF or dedicated health check ALBs, but the simplest and most secure approach leverages the fact that security groups are stateful and can reference each other. Remember the memory tip: “Chain the groups, not the rules”—the ALB group locks down to corporate IPs, and the instance group locks down to the ALB group, covering health checks automatically.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 runs a web application on EC2 instances in an Auto Scaling group behind an Application Load Balancer. The application must only accept traffic from known corporate IP addresses, but the company also needs to allow healthy traffic from AWS health checkers. Which architecture meets these requirements securely?
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
Configure the Application Load Balancer's security group to allow inbound from corporate IPs, and the EC2 instances' security group to allow inbound from the ALB's security group.
The correct approach is to use security groups. The ALB security group should allow inbound from corporate IPs, and the EC2 security group should allow inbound only from the ALB security group. AWS health checkers come from the ALB's private IPs, so they are covered. Option B is correct. Option A (NACL) is incorrect because NACLs are stateless and would require complex rules. Option C (WAF) can filter by IP but also needs to allow health checks, and it adds complexity. Option D (dedicated ALB for health checks) is overkill and unnecessary.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure the Application Load Balancer's security group to allow inbound from corporate IPs, and the EC2 instances' security group to allow inbound from the ALB's security group.
Why this is correct
This ensures that only traffic through the ALB (including health checks) reaches the instances, and the ALB only accepts corporate IP traffic.
Related concept
CIDR notation defines the prefix length.
- ✗
Create two target groups: one for corporate traffic and one for health check traffic, each with different security groups.
Why it's wrong here
Health checks are sent by the ALB itself, not by separate target groups; this approach doesn't solve the problem.
- ✗
Use AWS WAF on the ALB to create an IP set containing corporate IPs, and allow all traffic from ALB to instances.
Why it's wrong here
AWS WAF can filter by IP, but health checker traffic from the ALB is still allowed; however, WAF adds unnecessary cost and complexity compared to security groups.
- ✗
Use a network ACL on the VPC subnet to allow inbound traffic from corporate IPs and AWS health checker IP ranges.
Why it's wrong here
NACLs are stateless and would require explicitly allowing health checker traffic, but health checker IPs are not fixed; using security groups is simpler.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure the Application Load Balancer's security group to allow inbound from corporate IPs, and the EC2 instances' security group to allow inbound from the ALB's security group. — The correct approach is to use security groups. The ALB security group should allow inbound from corporate IPs, and the EC2 security group should allow inbound only from the ALB security group. AWS health checkers come from the ALB's private IPs, so they are covered. Option B is correct. Option A (NACL) is incorrect because NACLs are stateless and would require complex rules. Option C (WAF) can filter by IP but also needs to allow health checks, and it adds complexity. Option D (dedicated ALB for health checks) is overkill and unnecessary.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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