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Question 1,125 of 1,705

Network Implementation →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the on-premises DNS is resolving the EC2 instance’s public IP address instead of its private IP address. This is the most likely cause of suboptimal routing to the EC2 instance because when the on-premises router uses the public IP for destination, traffic must traverse the internet rather than the Direct Connect private VIF, which only handles private IP traffic. The core technical concept here is that DNS resolution dictates the path: if DNS returns a public IP, the router will route that traffic over the internet, even if a Direct Connect link is available and advertising the VPC CIDR. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how DNS integration with hybrid networking can override BGP routing decisions, a common trap where candidates assume BGP alone controls the path. A key memory tip is “DNS directs the destination: private IP for Direct Connect, public IP for internet.”

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 company has a Direct Connect connection with a private VIF to a VPC. The on-premises network team notices that traffic to a particular EC2 instance is taking a suboptimal path that goes through the internet instead of Direct Connect. The EC2 instance has both a private IP and a public IP. The on-premises router is advertising the VPC's CIDR block over BGP. 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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The private VIF is not configured to route the VPC CIDR.
Why wrong: The private VIF can route the VPC CIDR if advertised; the issue is that the on-premises device is using the public IP.
B
The security group on the EC2 instance is blocking traffic from the on-premises IP range.
Why wrong: Security groups control traffic at the instance level, but do not affect which path traffic takes.
C
The on-premises router is not advertising the VPC CIDR to the Direct Connect router.
Why wrong: The scenario states the on-premises router is advertising the VPC CIDR, so this is not the issue.
D
The on-premises DNS is resolving the EC2 instance's public IP address instead of its private IP address.
When using Direct Connect, you should use private IP addresses to ensure traffic stays on the private network.

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 on-premises DNS is resolving the EC2 instance's public IP address instead of its private IP address.

If the on-premises router uses the public IP to reach the instance, traffic will go over the internet, not Direct Connect. The issue is that the on-premises DNS or routing is resolving the public IP instead of the private IP for the instance. Option B is correct. Option A is incorrect because the private VIF is designed for private IP traffic. Option C is incorrect because security groups do not affect routing paths. Option D is incorrect because the BGP advertisement of the VPC CIDR is correct.

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 private VIF is not configured to route the VPC CIDR.
Why it's wrong here
The private VIF can route the VPC CIDR if advertised; the issue is that the on-premises device is using the public IP.
✗
The security group on the EC2 instance is blocking traffic from the on-premises IP range.
Why it's wrong here
Security groups control traffic at the instance level, but do not affect which path traffic takes.
✗
The on-premises router is not advertising the VPC CIDR to the Direct Connect router.
Why it's wrong here
The scenario states the on-premises router is advertising the VPC CIDR, so this is not the issue.
✓
The on-premises DNS is resolving the EC2 instance's public IP address instead of its private IP address.
Why this is correct
When using Direct Connect, you should use private IP addresses to ensure traffic stays on the private network.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.

Trap categories for this question

Scenario analysis trap
The scenario states the on-premises router is advertising the VPC CIDR, so this is not the issue.

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.

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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 on-premises DNS is resolving the EC2 instance's public IP address instead of its private IP address. — If the on-premises router uses the public IP to reach the instance, traffic will go over the internet, not Direct Connect. The issue is that the on-premises DNS or routing is resolving the public IP instead of the private IP for the instance. Option B is correct. Option A is incorrect because the private VIF is designed for private IP traffic. Option C is incorrect because security groups do not affect routing paths. Option D is incorrect because the BGP advertisement of the VPC CIDR is correct.

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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Last reviewed: Jun 20, 2026

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