The answer is a missing OS-level route for the second ENI’s subnet. When you attach a second elastic network interface to an EC2 instance, the operating system’s routing table still defaults to the primary ENI for all outbound traffic, regardless of which ENI sources the packets. If the instance sends traffic from the second ENI’s IP (10.0.2.100), the OS checks its routing table and finds no specific route for the 10.0.2.0/24 subnet tied to that ENI, so it sends the traffic out through the primary ENI’s default gateway—causing the outbound traffic to fail because the source IP doesn’t match the subnet of the egress interface. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how multi-ENI configurations interact with OS routing, a common trap where candidates blame subnet routing tables or security groups. Remember: AWS handles the network layer, but the OS must have explicit policy routing (e.g., ip rule and ip route commands) to direct traffic by source IP. Memory tip: “OS routes the source, not the subnet.”
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. They have two subnets: Subnet A (10.0.1.0/24) and Subnet B (10.0.2.0/24). They launch an EC2 instance in Subnet A and assign it a primary private IP address of 10.0.1.50. They then attach a second elastic network interface (ENI) to the instance with a primary private IP of 10.0.2.100. The instance needs to send traffic from the second ENI. However, when the instance sends traffic from the second ENI, it fails. 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.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance's operating system routing table does not have a route for the second ENI's subnet.
Option C is correct. When an EC2 instance has multiple ENIs, each ENI must be in a different subnet. However, the operating system's routing table sends traffic based on the source IP. If the default route is associated with the primary ENI, traffic sourced from the second ENI's IP will use the primary ENI's route, which may not allow it to leave through the correct subnet. The solution is to configure the OS to use the correct ENI for outbound traffic based on the source IP. Option A is incorrect because security groups are per-ENI and do not cause this issue. Option B is incorrect because there is no requirement for the ENIs to be in the same subnet. Option D is incorrect because the routing table in the subnet does not determine which ENI the instance uses.
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 two ENIs are in different subnets, which is not allowed.
Why it's wrong here
ENIs can be in different subnets.
✓
The instance's operating system routing table does not have a route for the second ENI's subnet.
Why this is correct
The OS routing table must have a route to send traffic from the second ENI through its own subnet.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
The security group for the second ENI does not allow outbound traffic.
Why it's wrong here
Security groups are stateful and allow outbound traffic by default.
✗
The subnet route table does not have a route for the second ENI's IP address.
Why it's wrong here
Subnet route tables handle traffic destined for other networks, not source-based routing.
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 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's operating system routing table does not have a route for the second ENI's subnet. — Option C is correct. When an EC2 instance has multiple ENIs, each ENI must be in a different subnet. However, the operating system's routing table sends traffic based on the source IP. If the default route is associated with the primary ENI, traffic sourced from the second ENI's IP will use the primary ENI's route, which may not allow it to leave through the correct subnet. The solution is to configure the OS to use the correct ENI for outbound traffic based on the source IP. Option A is incorrect because security groups are per-ENI and do not cause this issue. Option B is incorrect because there is no requirement for the ENIs to be in the same subnet. Option D is incorrect because the routing table in the subnet does not determine which ENI the instance uses.
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", "primary". 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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