The answer is that the EC2 instance is unreachable from the internet because it lacks a public IP address. Even though the security group and network ACL both permit inbound HTTP traffic from 0.0.0.0/0, and the subnet has a route to an internet gateway, the instance itself has no public IPv4 address or Elastic IP. For internet traffic to reach an EC2 instance in a VPC, the instance must have a public IP assigned at launch or an Elastic IP attached; without it, the internet gateway cannot map the destination address to the instance’s private IP. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental routing path: an internet gateway requires a public IP on the target instance to forward traffic, regardless of permissive security groups or ACLs. A common trap is assuming that a route to an IGW and open firewall rules are sufficient—they are not. Memory tip: “No public IP? No internet path.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 VPC with a CIDR block of 10.0.0.0/16. It has two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). Both subnets have a route to an Internet Gateway. An EC2 instance in subnet A has a security group that allows inbound HTTP from 0.0.0.0/0. The instance's network ACL allows inbound HTTP from 0.0.0.0/0. Users report they cannot access the web server. 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
✓
The instance does not have a public IP address
The instance does not have a public IP address, so it cannot be reached from the internet. Without a public IP or Elastic IP, traffic from the internet cannot be routed to the instance even if security groups and network ACLs allow it.
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 instance does not have a public IP address
Why this is correct
Without a public IP, the instance is not reachable from the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group is not associated with the instance
Why it's wrong here
The scenario states the security group allows inbound HTTP.
✗
The route table does not have a route to the Internet Gateway
Why it's wrong here
The scenario states both subnets have a route to the Internet Gateway.
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 security group allows inbound HTTP.
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.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instance does not have a public IP address — The instance does not have a public IP address, so it cannot be reached from the internet. Without a public IP or Elastic IP, traffic from the internet cannot be routed to the instance even if security groups and network ACLs allow it.
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.
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