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Question 810 of 1,705

Network Implementation →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the private VIF does not route traffic to public IP addresses, so on-premises traffic to the public IP must traverse the internet. A Direct Connect private virtual interface only forwards traffic destined for private IP ranges within the associated VPC; it has no mechanism to resolve or forward packets to public IPs, which are inherently internet-routable. Even though the EC2 instance has a public IP and a permissive security group, the private VIF’s routing table simply does not contain a path for public IP destinations, so the packets are dropped or never sent over the Direct Connect link. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental boundary between private VIFs and public connectivity—a common trap is assuming a security group or route table misconfiguration is the culprit. Remember the key distinction: private VIFs are for private IPs only; public IP reachability always requires a public VIF or an internet-based path. Memory tip: “Private VIF, private IPs only—public IPs need the public highway.”

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 virtual interface (VIF) to a VPC. The on-premises network team reports that they can ping the VPC's private IP addresses, but they cannot reach an internet-facing application hosted on an EC2 instance in a public subnet. The EC2 instance has a public IP and a security group allowing HTTP/HTTPS from 0.0.0.0/0. 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 1mediummultiple choice

Review the full subnetting walkthrough →

A
The route table in the public subnet does not have a default route to an internet gateway.
Why wrong: The application is reachable from the internet, so the route table must have a default route to the IGW.
B
The security group is blocking traffic from the on-premises IP range.
Why wrong: The security group allows all inbound HTTP/HTTPS from 0.0.0.0/0, so it is not blocking traffic.
C
The BGP session is not exchanging routes for the public IP range.
Why wrong: BGP exchanges private IP prefixes; public IPs are not advertised over private VIF.
D
The private VIF does not route traffic to public IP addresses; the on-premises traffic to the public IP must traverse the internet.
A private VIF only carries traffic destined to private IP addresses in the VPC. Traffic to public IPs must go over the internet.

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 private VIF does not route traffic to public IP addresses; the on-premises traffic to the public IP must traverse the internet.

The issue is that the private VIF uses private IP addresses, and traffic from on-premises to the public IP of the EC2 instance will not go over the private VIF; it must go over the internet. Option A is correct because the private VIF only routes private IPs. Option B is incorrect because the security group allows all inbound HTTP/HTTPS. Option C is incorrect because the route table can have a default route to an internet gateway. Option D is incorrect because BGP is typically used for prefix exchange and would not block traffic to public IPs.

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 route table in the public subnet does not have a default route to an internet gateway.
Why it's wrong here
The application is reachable from the internet, so the route table must have a default route to the IGW.
✗
The security group is blocking traffic from the on-premises IP range.
Why it's wrong here
The security group allows all inbound HTTP/HTTPS from 0.0.0.0/0, so it is not blocking traffic.
✗
The BGP session is not exchanging routes for the public IP range.
Why it's wrong here
BGP exchanges private IP prefixes; public IPs are not advertised over private VIF.
✓
The private VIF does not route traffic to public IP addresses; the on-premises traffic to the public IP must traverse the internet.
Why this is correct
A private VIF only carries traffic destined to private IP addresses in the VPC. Traffic to public IPs must go over the internet.
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.

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 private VIF does not route traffic to public IP addresses; the on-premises traffic to the public IP must traverse the internet. — The issue is that the private VIF uses private IP addresses, and traffic from on-premises to the public IP of the EC2 instance will not go over the private VIF; it must go over the internet. Option A is correct because the private VIF only routes private IPs. Option B is incorrect because the security group allows all inbound HTTP/HTTPS. Option C is incorrect because the route table can have a default route to an internet gateway. Option D is incorrect because BGP is typically used for prefix exchange and would not block traffic to public IPs.

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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