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 is designing a network architecture for a two-tier web application. The web tier runs on EC2 instances behind an Application Load Balancer (ALB) in public subnets. The application tier runs on EC2 instances in private subnets. The application tier needs to access an Amazon RDS for PostgreSQL database in the same private subnets. The company requires that all traffic between the ALB and web tier, as well as between web tier and application tier, remain within the AWS network and not traverse the internet. The current design uses an Internet Gateway (IGW) for public subnet internet access and a NAT Gateway for private subnet outbound internet access. The web tier instances have a default route to the IGW, and the application tier instances have a default route to the NAT Gateway. The security groups are configured correctly. However, the application tier cannot connect to the RDS database. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The application tier instances are using the RDS public DNS name instead of the private DNS name
Using the public endpoint causes traffic to go to the NAT Gateway, which cannot connect to RDS's public endpoint from private subnet without proper routing.
B
The RDS database is in a different VPC
Why wrong: The description says same VPC.
C
The ALB is not configured to forward traffic to the web tier
Why wrong: The issue is between application tier and RDS, not ALB to web.
D
The NAT Gateway is not configured with the correct route to the RDS subnet
Why wrong: NAT Gateway is for internet-bound traffic; RDS is in the same VPC, so routing is via local routes.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The application tier instances are using the RDS public DNS name instead of the private DNS name
The RDS database is in private subnets. The application tier instances are also in private subnets. They should be able to communicate within the same VPC via private IP addresses. The issue is not about internet access. The most likely cause is that the application tier instances are trying to connect to the RDS endpoint using the public DNS name, which resolves to a public IP, and the traffic is being routed to the NATGateway, which blocks inbound traffic from the internet (the RDS public endpoint). The application tier should use the private DNS name or the private IP address of the RDS instance. Alternatively, the security group might be misconfigured, but the question says security groups are correct. The most common mistake is using the public endpoint.
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.
✓
The application tier instances are using the RDS public DNS name instead of the private DNS name
Why this is correct
Using the public endpoint causes traffic to go to the NATGateway, which cannot connect to RDS's public endpoint from private subnet without proper routing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The ALB is not configured to forward traffic to the web tier
Why it's wrong here
The issue is between application tier and RDS, not ALB to web.
✗
The NAT Gateway is not configured with the correct route to the RDS subnet
Why it's wrong here
NATGateway is for internet-bound traffic; RDS is in the same VPC, so routing is via local routes.
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.
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: The application tier instances are using the RDS public DNS name instead of the private DNS name — The RDS database is in private subnets. The application tier instances are also in private subnets. They should be able to communicate within the same VPC via private IP addresses. The issue is not about internet access. The most likely cause is that the application tier instances are trying to connect to the RDS endpoint using the public DNS name, which resolves to a public IP, and the traffic is being routed to the NAT Gateway, which blocks inbound traffic from the internet (the RDS public endpoint). The application tier should use the private DNS name or the private IP address of the RDS instance. Alternatively, the security group might be misconfigured, but the question says security groups are correct. The most common mistake is using the public endpoint.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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