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Question 37 of 1,705

Network Design →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is missing route table entries for the database subnet CIDRs in the application subnets' route tables. This is the most likely cause because when an RDS Multi-AZ deployment spans subnets in different Availability Zones, the DNS endpoint can resolve to a primary instance in a database subnet that resides in a different AZ than the application tier. If the application subnet's route table lacks an explicit route to the database subnet's CIDR block, traffic to that IP address will be dropped, causing intermittent timeouts—even though security groups and network ACLs are correctly configured. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how RDS DNS resolution interacts with subnet routing in a multi-AZ architecture, a common trap where engineers focus only on security groups and forget that subnets in different AZs are separate networks requiring explicit routes. A key memory tip: RDS endpoints are DNS-based, not IP-based; always verify that every application subnet has a route to every database subnet CIDR in the RDS subnet group.

ANS-C01 Network Design Practice Question

This ANS-C01 practice question tests your understanding of network design. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 deployed a multi-tier web application in a single AWS region. The architecture includes a VPC with public and private subnets across two Availability Zones. The web tier uses an Application Load Balancer (ALB) in the public subnets, and the application tier runs on EC2 instances in the private subnets. The database tier uses an Amazon RDS Multi-AZ deployment in the database subnets. The company is experiencing intermittent connectivity issues between the application tier and the database tier. The application logs show connection timeouts. The network engineer has verified that the security groups and network ACLs are correctly configured. The RDS instance is reachable from the application tier via a telnet test from one specific instance, but not consistently from all instances. What is the most likely cause of the intermittent connectivity?

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.

Question 1mediummultiple choice

Study the full ACL explanation →

A
The RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint.
Why wrong: Wrong: Multi-AZ failover changes DNS, but DNS TTL should be short; intermittent issues suggest routing.
B
The network ACLs on the database subnets are blocking ephemeral ports used by the application.
Why wrong: Wrong: Network ACLs are stateless; ephemeral ports should be allowed outbound.
C
The database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs.
Correct: Missing routes cause intermittent connectivity depending on which AZ the instance is in.
D
The security group for the database is allowing traffic only from the application tier's security group, but the application tier instances are using a different security group.
Why wrong: Wrong: If security groups are correct, traffic should be allowed.

Full breakdown with real-world context →

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

✓

The database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs.

Option B is correct because if the RDS subnet group spans subnets in different AZs, the RDS endpoint may resolve to an IP in a different subnet that is not routable due to missing route table entries. Option A is wrong because RDS Multi-AZ does not use a separate IP for failover; the DNS CNAME changes. Option C is wrong because security groups are stateful and allow return traffic. Option D is wrong because network ACLs are stateless but if configured correctly they should allow 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.

✗
The RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint.
Why it's wrong here
Wrong: Multi-AZ failover changes DNS, but DNS TTL should be short; intermittent issues suggest routing.
✗
The network ACLs on the database subnets are blocking ephemeral ports used by the application.
Why it's wrong here
Wrong: Network ACLs are stateless; ephemeral ports should be allowed outbound.
✓
The database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs.
Why this is correct
Correct: Missing routes cause intermittent connectivity depending on which AZ the instance is in.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The security group for the database is allowing traffic only from the application tier's security group, but the application tier instances are using a different security group.
Why it's wrong here
Wrong: If security groups are correct, traffic should be allowed.

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 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 database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs. — Option B is correct because if the RDS subnet group spans subnets in different AZs, the RDS endpoint may resolve to an IP in a different subnet that is not routable due to missing route table entries. Option A is wrong because RDS Multi-AZ does not use a separate IP for failover; the DNS CNAME changes. Option C is wrong because security groups are stateful and allow return traffic. Option D is wrong because network ACLs are stateless but if configured correctly they should allow traffic.

What should I do if I get this ANS-C01 question wrong?

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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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 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?

medium

✓ A.The application tier instances are using the RDS public DNS name instead of the private DNS name
B.The RDS database is in a different VPC
C.The ALB is not configured to forward traffic to the web tier
D.The NAT Gateway is not configured with the correct route to the RDS subnet

Why A: 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.

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