The correct approach is to create separate subnets for dev and prod and use network ACLs to allow only specific ports between them, because network ACLs provide stateless, subnet-level filtering that can isolate environments within the same VPC while permitting limited, controlled communication. This works because NACLs evaluate traffic in both directions independently, so you can explicitly allow only the required ports inbound and outbound between the dev and prod subnets, blocking all other traffic by default. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of when to use stateless versus stateful filtering, and a common trap is choosing security groups—which are stateful and cannot easily block return traffic—or assuming separate VPCs are necessary, which adds unnecessary complexity. Remember the mnemonic: “NACLs are for networks, SGs are for servers”—if you need to isolate entire subnets within the same VPC, always reach for the stateless NACL.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 large VPC with multiple workloads. They need to isolate development and production environments within the same VPC, but allow limited communication between them via specific ports. Which approach meets these requirements?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create separate subnets for dev and prod, use network ACLs to allow only specific ports between them.
Option B is correct because network ACLs are stateless and can be used to allow specific traffic between subnets while keeping subnets in separate networks. Option A is wrong because separate VPCs require peering or other connections, adding complexity. Option C is wrong because security groups are stateful and cannot be used to block return traffic easily. Option D is wrong because NACLs on the same subnet cannot isolate environments within the same subnet.
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.
✗
Place dev and prod in the same subnet and use security groups to filter traffic.
Why it's wrong here
Security groups cannot isolate traffic within the same subnet; they apply to instances.
✗
Use a single subnet and rely on network ACLs to separate traffic.
Why it's wrong here
Network ACLs are applied at subnet boundaries, not within a subnet.
✓
Create separate subnets for dev and prod, use network ACLs to allow only specific ports between them.
Why this is correct
Network ACLs are stateless and can control traffic between subnets effectively.
Create two VPCs and use VPC peering to allow communication.
Why it's wrong here
This adds complexity and is not necessary if isolation can be achieved within one VPC.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create separate subnets for dev and prod, use network ACLs to allow only specific ports between them. — Option B is correct because network ACLs are stateless and can be used to allow specific traffic between subnets while keeping subnets in separate networks. Option A is wrong because separate VPCs require peering or other connections, adding complexity. Option C is wrong because security groups are stateful and cannot be used to block return traffic easily. Option D is wrong because NACLs on the same subnet cannot isolate environments within the same subnet.
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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Question Discussion
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