CompTIA Network+ (N10-009)Network TroubleshootingMedium
A network administrator is investigating a report that users on a specific subnet (192.168.10.0/24) can send traffic to a web server (172.16.1.100) located in a different subnet, but they never receive a response. The web server itself is functioning correctly and can communicate with other subnets. Which of the following is the MOST likely cause of this asymmetric communication block?
- AA firewall is blocking return traffic from the web server to the 192.168.10.0/24 subnet.
- BThe web server's default gateway is incorrectly configured.
- CThe web server has a duplicate IP address.
- DThe 192.168.10.0/24 subnet's router has an incorrect route to the web server.
Show answer & explanationAnswer & explanation
Correct answer: A. A firewall is blocking return traffic from the web server to the 192.168.10.0/24 subnet.
If users can send traffic but not receive a response, it implies the outbound path is working but the return path is blocked. Since the web server can communicate with other subnets, its own gateway and IP are likely correct. A firewall blocking the return traffic specifically to the users' subnet is a common cause of this asymmetric communication.
Why the other options are wrong
- B. If the web server's default gateway were incorrect, it wouldn't be able to communicate with *any* other subnets, contradicting the problem statement.
- C. A duplicate IP address on the web server would cause intermittent or complete loss of all communication, not just one-way communication to a specific subnet.
- D. If the users' router had an incorrect route, the initial outbound traffic from 192.168.10.0/24 to 172.16.1.100 would not reach the web server at all.
Asymmetric Routing/Communication Block
A network condition where traffic flows successfully in one direction but fails or is blocked in the reverse direction, often due to security devices or misconfigurations.
- Outbound traffic succeeds, inbound fails.
- Commonly caused by stateful firewalls blocking uninitiated return traffic.
- Can also be caused by routing issues where return path is unknown or suboptimal.
Memory trick: Traffic goes out, but doesn't come back in; a firewall's rule might be the sin.