ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
Many candidates assume the issue is with AWS networking components (security groups or route tables) rather than the instance's operating system routing, which is a common oversight in multi-ENI configurations.
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 instance's operating system routing table does not have a route for the second ENI's subnet.
When an EC2 instance has multiple ENIs, the operating system's routing table must have explicit routes for each ENI's subnet to ensure traffic leaves via the correct interface. By default, the OS only adds a route for the primary ENI's subnet, so traffic from the second ENI (10.0.2.100) fails because the OS tries to send it out the primary ENI, which is in a different subnet. This is a common cause of asymmetric routing or connectivity failure.
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.
- ✗
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.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.