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Network Implementation →mediumMultiple ChoiceObjective-mapped

ANS-C01 Network Implementation Practice Question

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 network engineer is troubleshooting connectivity issues from an on-premises data center to an Amazon VPC via an AWS Site-to-Site VPN. The VPN tunnel is up, but ping from an on-premises host (10.0.0.5) to an EC2 instance (172.16.1.10) fails. The VPC CIDR is 172.16.0.0/16. The on-premises CIDR is 10.0.0.0/8. The customer gateway device has a route for 172.16.0.0/16 pointing to the VPN tunnel. The VPC route table has a route for 10.0.0.0/8 pointing to the virtual private gateway. Security groups and NACLs allow ICMP. 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.

A
The on-premises host does not have a route to the VPC CIDR
Why wrong: Incorrect; the on-premises host has a route for 172.16.0.0/16 pointing to the VPN tunnel.
B
The customer gateway device is not configured with the correct pre-shared key
Why wrong: Incorrect; if the tunnel is up, the pre-shared key is correct.
C
The route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR
Correct; the subnet route table must include a route for the on-premises CIDR pointing to the virtual private gateway.
D
The virtual private gateway is not attached to the correct VPC
Why wrong: Incorrect; the VPC route table has a route to the VGW, so it is attached.

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 route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR

The most likely cause is that the subnet route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR (10.0.0.0/8). Although the VPC's main route table has a route for 10.0.0.0/8 pointing to the virtual private gateway, the EC2 instance's subnet may be using a custom route table that lacks this route. Without it, the EC2 instance's return ICMP traffic to 10.0.0.5 is sent via the local route (which only covers the VPC CIDR 172.16.0.0/16) and does not reach the VPN tunnel, causing the ping to fail.

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 on-premises host does not have a route to the VPC CIDR
Why it's wrong here
Incorrect; the on-premises host has a route for 172.16.0.0/16 pointing to the VPN tunnel.
✗
The customer gateway device is not configured with the correct pre-shared key
Why it's wrong here
Incorrect; if the tunnel is up, the pre-shared key is correct.
✓
The route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR
Why this is correct
Correct; the subnet route table must include a route for the on-premises CIDR pointing to the virtual private gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The virtual private gateway is not attached to the correct VPC
Why it's wrong here
Incorrect; the VPC route table has a route to the VGW, so it is attached.

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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.

Visual reference

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: The route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR — The most likely cause is that the subnet route table associated with the EC2 instance's subnet does not have a route for the on-premises CIDR (10.0.0.0/8). Although the VPC's main route table has a route for 10.0.0.0/8 pointing to the virtual private gateway, the EC2 instance's subnet may be using a custom route table that lacks this route. Without it, the EC2 instance's return ICMP traffic to 10.0.0.5 is sent via the local route (which only covers the VPC CIDR 172.16.0.0/16) and does not reach the VPN tunnel, causing the ping to fail.

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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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.