The answer is to allow inbound traffic from the ALB subnet CIDR on port 8080 and ephemeral ports, and outbound traffic to the database subnet CIDR on port 3306. This is correct because the private subnet web servers only need to receive forwarded HTTPS traffic from the ALB on port 8080, while ephemeral ports must be open inbound to allow return traffic for established connections. The outbound rule then permits the web servers to query the RDS database on port 3306, completing the three-tier architecture. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateful versus stateless filtering—network ACLs are stateless, so you must explicitly allow both inbound and outbound traffic for each flow. A common trap is confusing security group rules with NACL rules, leading candidates to mistakenly open inbound port 443 or 3306 on the private subnet. Remember the memory tip: “NACL is stateless, so think in pairs—inbound for the listener, outbound for the talker.”
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 security engineer is designing a network ACL for a public-facing web application in a VPC. The application uses an Application Load Balancer (ALB) in a public subnet, web servers in private subnets, and an RDS database in a private subnet. The engineer needs to allow HTTPS traffic from the internet to the ALB, and allow the ALB to forward traffic to the web servers on port 8080. The web servers need to query the database on port 3306. Which network ACL configuration should the engineer implement for the private subnet containing the web servers?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inbound: allow ALB subnet CIDR on port 8080 and ephemeral ports; Outbound: allow database subnet CIDR on port 3306
Option C is correct because inbound allows ALB traffic on port 8080 and ephemeral ports for responses, outbound allows database traffic. Option A is wrong because web servers do not need inbound on port 443. Option B is wrong because inbound port 3306 is not needed. Option D is wrong because inbound should not allow all traffic.
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.
✗
Inbound: allow 0.0.0.0/0 on port 3306; Outbound: allow 0.0.0.0/0 on port 8080
Why it's wrong here
Database port should be outbound, not inbound.
✗
Inbound: allow 0.0.0.0/0 on port 8080; Outbound: allow 0.0.0.0/0 on all ports
Why it's wrong here
Too permissive outbound.
✓
Inbound: allow ALB subnet CIDR on port 8080 and ephemeral ports; Outbound: allow database subnet CIDR on port 3306
Why this is correct
Correctly restricts traffic to necessary ports and sources.
Inbound: allow 0.0.0.0/0 on port 443; Outbound: allow 0.0.0.0/0 on port 3306
Why it's wrong here
Inbound should be from ALB on port 8080.
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.
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: Inbound: allow ALB subnet CIDR on port 8080 and ephemeral ports; Outbound: allow database subnet CIDR on port 3306 — Option C is correct because inbound allows ALB traffic on port 8080 and ephemeral ports for responses, outbound allows database traffic. Option A is wrong because web servers do not need inbound on port 443. Option B is wrong because inbound port 3306 is not needed. Option D is wrong because inbound should not allow all traffic.
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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