ANS-C01 Network Implementation Practice Question
A company has a VPC with CIDR 10.0.0.0/16. They have a public subnet (10.0.1.0/24) and a private subnet (10.0.2.0/24). They have a Network Load Balancer (NLB) in the public subnet, and EC2 instances in the private subnet. The NLB has a target group pointing to the EC2 instances. The security group for the EC2 instances allows traffic from the NLB's private IP addresses on port 80. The network ACL for the private subnet allows inbound traffic on port 80 from the public subnet CIDR and outbound ephemeral ports to 0.0.0.0/0. However, clients connecting to the NLB experience intermittent timeouts. The network engineer checks the NLB logs and sees that connections are established but occasionally drop. The engineer also notices that the EC2 instances have a default route to a NAT Gateway in the public subnet. What is the most likely cause of the intermittent timeouts?
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 EC2 instances have a default route to the NAT Gateway, causing asymmetric routing.
The NLB with client IP preservation sends traffic to targets with the client's source IP. The EC2 instances have a default route pointing to the NAT Gateway. When the instance responds to a client, the response is routed to the NAT Gateway (because the destination is the client's IP, which is outside the VPC). The NAT Gateway then sends the response to the internet, but the client expects the response from the NLB's IP address. This asymmetric routing causes the NLB to not see the response, leading to intermittent timeouts. Options A, B, and D are incorrect: disabling cross-zone load balancing does not cause timeouts, health checks are independent, and NLB does not have security groups.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Cross-zone load balancing is disabled on the NLB.
Why it's wrong here
This would cause uneven distribution but not timeouts.
- ✗
The NLB health checks are failing, causing targets to be removed.
Why it's wrong here
This would cause complete failure, not intermittent.
- ✓
The EC2 instances have a default route to the NAT Gateway, causing asymmetric routing.
Why this is correct
Return traffic goes to NAT instead of NLB, breaking the connection.
- ✗
The security group for the NLB is blocking inbound traffic.
Why it's wrong here
NLBs do not have security groups.
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
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