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Question 339 of 1,705

Network Management and Operations →hardMultiple ChoiceObjective-mapped

VPC Peering Security Group Blocking Specific Ports

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 network engineer is troubleshooting connectivity issues between two VPCs that are connected via VPC peering. The VPCs are in the same region and have overlapping CIDR blocks. The engineer can ping the private IP of an instance in the peered VPC from an instance in the first VPC. However, traffic on TCP port 443 (HTTPS) fails. Which 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: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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 network ACL in the target subnet is blocking inbound HTTPS traffic
Why wrong: NACLs are stateless; if they block HTTPS, they would likely block ICMP as well unless specifically allowed.
B
The security group of the target instance does not allow inbound HTTPS traffic from the source
Security groups are stateful; ICMP may be allowed but HTTPS not.
C
The VPC peering connection is not in the 'active' state
Why wrong: Ping works, so connection is active.
D
The route tables in both VPCs do not have routes to the peered VPC's CIDR
Why wrong: If ping works, routing is correct.

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 security group of the target instance does not allow inbound HTTPS traffic from the source

The correct answer is B. The security group of the target instance must allow inbound HTTPS traffic from the source instance's security group or CIDR. Option A is incorrect because ICMP works, indicating that network ACLs are allowing traffic; network ACLs are stateless and would block ICMP if misconfigured. Option C is incorrect because the VPC peering connection must be active for any traffic to pass, and ICMP works. Option D is incorrect because route tables must have routes to the peered VPC's CIDR for traffic to flow, and ICMP works.

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 network ACL in the target subnet is blocking inbound HTTPS traffic
Why it's wrong here
NACLs are stateless; if they block HTTPS, they would likely block ICMP as well unless specifically allowed.
✓
The security group of the target instance does not allow inbound HTTPS traffic from the source
Why this is correct
Security groups are stateful; ICMP may be allowed but HTTPS not.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The VPC peering connection is not in the 'active' state
Why it's wrong here
Ping works, so connection is active.
✗
The route tables in both VPCs do not have routes to the peered VPC's CIDR
Why it's wrong here
If ping works, routing is correct.

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.

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 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 security group of the target instance does not allow inbound HTTPS traffic from the source — The correct answer is B. The security group of the target instance must allow inbound HTTPS traffic from the source instance's security group or CIDR. Option A is incorrect because ICMP works, indicating that network ACLs are allowing traffic; network ACLs are stateless and would block ICMP if misconfigured. Option C is incorrect because the VPC peering connection must be active for any traffic to pass, and ICMP works. Option D is incorrect because route tables must have routes to the peered VPC's CIDR for traffic to flow, and ICMP works.

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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

CIDR notation defines the prefix length.

About these practice questions

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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 network engineer is troubleshooting intermittent connectivity issues between two VPCs connected via a VPC peering connection. The engineer notices that the route tables in both VPCs have the correct routes. What should the engineer check next?

easy

✓ A.Check security group and network ACL rules
B.Verify that DNS resolution is enabled for the VPCs
C.Ensure that the VPN connection is active
D.Check the internet gateway configuration

Why A: Option A is correct because security group rules and NACLs can block traffic even if routes are correct. Option B is wrong because DNS resolution is not related to basic connectivity. Option C is wrong because internet gateway is not involved in VPC peering. Option D is wrong because VPN connection is a different service.

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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.