- A
Use network ACLs on the application tier subnets to allow inbound from web tier and block outbound to web tier
Why wrong: Network ACLs are stateless; blocking outbound would prevent responses.
- B
Use a transit gateway with route tables to control traffic flow
Why wrong: Transit gateway does not provide stateful filtering.
- C
Place a reverse proxy between the tiers
Why wrong: Unnecessary; security groups suffice.
- D
Use security groups on the application tier instances to allow inbound from the web tier security group, and do not allow inbound from application tier in the web tier security group
Stateful security groups allow responses and block unwanted initiation.
Question 1,380 of 1,705
Using Security Group References to Allow One-Way Communication Between Application Tiers
This ANS-C01 practice question tests your understanding of network implementation. 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 is implementing a network for a three-tier application in a VPC. They need to ensure that the web tier can communicate with the application tier, but the application tier cannot initiate connections to the web tier. Which configuration should be used?
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
✓
Use security groups on the application tier instances to allow inbound from the web tier security group, and do not allow inbound from application tier in the web tier security group
Option D is correct because security groups are stateful and can be configured to allow inbound traffic from the web tier security group to the application tier instances, while the web tier security group only allows inbound from the application tier's security group on specific ports (if needed) and does not allow inbound from the application tier. This prevents the application tier from initiating connections to the web tier. Option A is wrong because network ACLs are stateless and would require explicit inbound and outbound rules to control the direction, but they do not track connection state and are more complex to configure for this requirement. Option B is wrong because a transit gateway is used for connecting multiple VPCs or on-premises networks, not for controlling traffic direction between tiers within the same VPC. Option C is wrong because a reverse proxy is used to forward requests from clients to servers, not to restrict initiation of connections.
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.
- ✗
Use network ACLs on the application tier subnets to allow inbound from web tier and block outbound to web tier
Why it's wrong here
Network ACLs are stateless; blocking outbound would prevent responses.
- ✗
Use a transit gateway with route tables to control traffic flow
Why it's wrong here
Transit gateway does not provide stateful filtering.
- ✗
Place a reverse proxy between the tiers
Why it's wrong here
Unnecessary; security groups suffice.
- ✓
Use security groups on the application tier instances to allow inbound from the web tier security group, and do not allow inbound from application tier in the web tier security group
Why this is correct
Stateful security groups allow responses and block unwanted initiation.
Related concept
CIDR notation defines the prefix length.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Use security groups on the application tier instances to allow inbound from the web tier security group, and do not allow inbound from application tier in the web tier security group — Option D is correct because security groups are stateful and can be configured to allow inbound traffic from the web tier security group to the application tier instances, while the web tier security group only allows inbound from the application tier's security group on specific ports (if needed) and does not allow inbound from the application tier. This prevents the application tier from initiating connections to the web tier. Option A is wrong because network ACLs are stateless and would require explicit inbound and outbound rules to control the direction, but they do not track connection state and are more complex to configure for this requirement. Option B is wrong because a transit gateway is used for connecting multiple VPCs or on-premises networks, not for controlling traffic direction between tiers within the same VPC. Option C is wrong because a reverse proxy is used to forward requests from clients to servers, not to restrict initiation of connections.
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.
About these practice questions
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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 is designing a multi-tier application with web servers in public subnets and database servers in private subnets. The database servers should only be accessible from the web servers. Which AWS feature should be used to enforce this?
easy- ✓ A.Security groups that reference the web server security group as a source
- B.VPC endpoints to restrict access to the database servers
- C.Network ACLs with allow rules for the web server subnet CIDR
- D.Network ACLs with deny rules for all traffic except from the web server subnet
Why A: Security groups are stateful and act as a virtual firewall for individual instances. By referencing the web server security group as a source in the database security group's inbound rules, you allow traffic only from instances in that security group. This provides granular control at the instance level. Option B is incorrect because VPC endpoints are used to privately connect to AWS services, not to restrict database access. Options C and D are incorrect because network ACLs are stateless and applied at the subnet level, not at the instance level, and they require explicit allow rules for return traffic. Using a NACL with allow rules for the web server subnet CIDR would still allow any instance in that subnet, not just the web servers, and would require managing ephemeral ports.
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Last reviewed: Jun 20, 2026
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