The answer is a security group or network ACL blocking inbound TCP port 443 on the EC2 instance. This is correct because security groups and network ACLs operate at different layers of the OSI model—security groups act as a stateful firewall at the instance level, while network ACLs provide stateless filtering at the subnet level—and both can explicitly permit ICMP traffic (used by ping) while denying TCP traffic on specific ports like HTTPS. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how stateful versus stateless filtering interacts with VPN connectivity, and it’s a common trap to assume that a working ping implies all traffic is allowed; in reality, ICMP and TCP are evaluated independently by separate rules. A key memory tip is “ping is not a protocol test”—ICMP success only proves Layer 3 reachability, not Layer 4 port access, so always verify security group and ACL inbound rules for the specific TCP port.
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 network engineer is troubleshooting connectivity issues between an EC2 instance in a VPC and an on-premises server over AWS Site-to-Site VPN. The VPN tunnel status is UP, and BGP is established. The engineer can ping the on-premises server's private IP from the EC2 instance, but TCP connections to a specific port (e.g., 443) are timing out. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Security group or network ACL blocking inbound TCP port 443 on the EC2 instance
Option B is correct because security groups or network ACLs can block specific ports while allowing ICMP. Option A is wrong because MTU mismatch would affect all traffic. Option C is wrong because the VPN tunnel is UP, so routing issues would affect all traffic. Option D is wrong because BGP is established, indicating proper route exchange.
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.
✗
Incorrect route table entries on the on-premises router for the VPC CIDR
Why it's wrong here
Incorrect routes would cause ping failure as well.
✓
Security group or network ACL blocking inbound TCP port 443 on the EC2 instance
Why this is correct
Stateful firewalls can block specific ports while allowing ICMP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
BGP is established, so prefix limits are likely not an issue.
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.
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: Security group or network ACL blocking inbound TCP port 443 on the EC2 instance — Option B is correct because security groups or network ACLs can block specific ports while allowing ICMP. Option A is wrong because MTU mismatch would affect all traffic. Option C is wrong because the VPN tunnel is UP, so routing issues would affect all traffic. Option D is wrong because BGP is established, indicating proper route exchange.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 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?
hard
A.The network ACL in the target subnet is blocking inbound HTTPS traffic
✓ B.The security group of the target instance does not allow inbound HTTPS traffic from the source
C.The VPC peering connection is not in the 'active' state
D.The route tables in both VPCs do not have routes to the peered VPC's CIDR
Why B: Option A is correct because the security group of the target instance must allow inbound HTTPS traffic from the source instance's security group or CIDR. Option B is wrong because ICMP works. Option C is wrong because the route table has a route to the peering connection. Option D is wrong because NACLs are stateless and would block ICMP too if misconfigured.
Variation 2. 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.
Last reviewed: Jun 20, 2026
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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.
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