- A
Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0
Why wrong: While possible, the security group already provides this; NACL is optional.
- B
Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0 and outbound ephemeral ports
NACL on public subnet provides stateless filtering for inbound HTTPS and outbound responses.
- C
Network ACL on the private subnet allowing inbound 3306 from the public subnet CIDR
Why wrong: NACL is stateless and can be used, but the security group is the recommended control.
- D
Security group for application servers allowing inbound 3306 from the web server security group
This ensures only web servers can access the application servers on the database port.
- E
Security group for web servers allowing inbound 443 from 0.0.0.0/0
This allows HTTPS traffic from the internet to the web servers.
Quick Answer
The correct answer involves configuring a security group for the web servers to allow inbound HTTPS on port 443 from 0.0.0.0/0, a security group for the application servers to allow inbound MySQL on port 3306 from the web server security group, and a network ACL on the public subnet for an additional stateless filtering layer. This design leverages security groups as stateful, instance-level firewalls that track connection state, making them ideal for controlling traffic between tiers, while the stateless NACL on the public subnet provides a subnet-level barrier against unsolicited inbound traffic. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the nuanced interplay between security groups and NACLs in a multi-tier web application, often with a trap where candidates mistakenly apply a NACL to the private subnet instead of relying on the security group for primary access control. Remember the memory tip: "Stateful SGs for instance control, stateless NACLs for subnet patrol"—use NACLs only where you need explicit allow/deny rules at the subnet boundary, not for internal tier-to-tier filtering.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 is designing a network security solution for a VPC that hosts a multi-tier web application. The web servers are in a public subnet, and the application servers are in a private subnet. The company wants to ensure that the web servers can only be accessed on port 443 from the internet, and the application servers can only be accessed from the web servers on port 3306. Which THREE components should be configured?
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
Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0 and outbound ephemeral ports
Option A (Security group for web servers) allows inbound 443 from internet. Option D (Security group for application servers) allows inbound 3306 from web server security group. Option E (Network ACL on public subnet) can add a layer of stateless filtering. Option B is wrong because a NACL on private subnet would allow all traffic unless explicitly denied, but the security group is the primary control. Option C is wrong because a NACL on the public subnet is acceptable, but for the private subnet, the security group is sufficient, and NACL is not required.
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.
- ✗
Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0
Why it's wrong here
While possible, the security group already provides this; NACL is optional.
- ✓
Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0 and outbound ephemeral ports
- ✗
Network ACL on the private subnet allowing inbound 3306 from the public subnet CIDR
Why it's wrong here
NACL is stateless and can be used, but the security group is the recommended control.
- ✓
Security group for application servers allowing inbound 3306 from the web server security group
Why this is correct
This ensures only web servers can access the application servers on the database port.
Related concept
CIDR notation defines the prefix length.
- ✓
Security group for web servers allowing inbound 443 from 0.0.0.0/0
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: Network ACL on the public subnet allowing inbound 443 from 0.0.0.0/0 and outbound ephemeral ports — Option A (Security group for web servers) allows inbound 443 from internet. Option D (Security group for application servers) allows inbound 3306 from web server security group. Option E (Network ACL on public subnet) can add a layer of stateless filtering. Option B is wrong because a NACL on private subnet would allow all traffic unless explicitly denied, but the security group is the primary control. Option C is wrong because a NACL on the public subnet is acceptable, but for the private subnet, the security group is sufficient, and NACL is not required.
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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