The answer is that both route tables are the same resource due to a typo in the template. This is the most likely cause of the CloudFormation duplicate default route error because the error message explicitly states that route table rtb-12345678 already has a route to 0.0.0.0/0, meaning the same physical route table is being targeted by both the public and private route definitions. In CloudFormation, if a logical resource name is misspelled or accidentally reused, both route entries will reference the same underlying route table ID, causing the duplicate default route conflict. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how CloudFormation resource references work and how subtle typos can break infrastructure deployments. A common trap is assuming the error is about route table limits or gateway conflicts, but the key is that a single route table can only have one default route (0.0.0.0/0). Memory tip: one route table, one default route—if you see a duplicate, check your resource names for a typo.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 using AWS CloudFormation to deploy a VPC with public and private subnets, an internet gateway, and a NAT gateway. The deployment fails with the error: 'Route table rtb-12345678 already has a route with destination 0.0.0.0/0'. The network engineer reviews the template and sees that the public route table has a route to the internet gateway and the private route table has a route to the NAT gateway. Both route tables are defined in the same template. What is the most likely cause of this error?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The public and private route tables are the same resource due to a typo in the template.
Option B is correct because the error indicates that the same route table is being used for both routes, which happens when both resources reference the same route table ID. Option A is wrong because a route table can have multiple routes, but only one default route. Option C is wrong because the error is specific to duplicate routes, not a missing route. Option D is wrong because internet and NAT gateways can coexist in the same VPC.
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 internet gateway and NAT gateway cannot exist in the same VPC simultaneously.
Why it's wrong here
Incorrect: They can coexist; one is for public subnets, the other for private.
✓
The public and private route tables are the same resource due to a typo in the template.
Why this is correct
Correct: Using the same route table ID for both routes causes duplicate route error.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The public route table already has a route to the internet gateway, and adding another default route is not allowed.
Why it's wrong here
Incorrect: A route table can have only one default route, but the error shows the same route table is being used.
✗
The NAT gateway route is missing a destination CIDR block.
Why it's wrong here
Incorrect: The error is about duplicate route, not missing destination.
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.
Trap categories for this question
Command / output trap
Incorrect: A route table can have only one default route, but the error shows the same route table is being used.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The public and private route tables are the same resource due to a typo in the template. — Option B is correct because the error indicates that the same route table is being used for both routes, which happens when both resources reference the same route table ID. Option A is wrong because a route table can have multiple routes, but only one default route. Option C is wrong because the error is specific to duplicate routes, not a missing route. Option D is wrong because internet and NAT gateways can coexist in the same VPC.
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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