Question

Difficulty: MediumTroubleshooting Routing, Default Gateways, and ACLs

A network technician is provisioning a new telemetry server at a branch office. The server is assigned the IPv4 address 192.168.48.150/25192.168.48.150/25. Local testing shows that the server cannot reach external network services or ping its configured gateway. A review of the network configuration details reveals:

- Server IP Address: 192.168.48.150192.168.48.150
- Subnet Mask: 255.255.255.128255.255.255.128 (/25/25)
- Default Gateway: 192.168.48.1192.168.48.1
- Router Interface IP: 192.168.48.1/25192.168.48.1/25

Which of the following identifies the root cause of the connectivity failure?

  1. The default gateway IP address resides on a different IP subnet than the server's configured address.Answer
  2. B
    The host IP address 192.168.48.150 is reserved as the broadcast address for the /25 network.
  3. C
    An access control list on the router is dropping traffic due to an implicit deny rule.
  4. D
    The host and router are on different VLANs and require a Spanning Tree protocol update to communicate.

Answer

The server's configured default gateway address resides on a different IP subnet than the host address itself.
A host device requires its default gateway IP address to reside within the same local IP subnet so it can discover the gateway's MAC address via ARP. With a /25 subnet mask (255.255.255.128), the network is split into two subnets: 192.168.48.0/25 (192.168.48.0 - 192.168.48.127) and 192.168.48.128/25 (192.168.48.128 - 192.168.48.255). The server's IP address (192.168.48.150) is in the second subnet, while the default gateway (192.168.48.1) is in the first subnet. Because they are on separate subnets, the host cannot route traffic to external destinations.

Step-by-Step Solution

1
Calculate the IP subnet parameters for the server's IP address.
IP 192.168.48.150 with mask 255.255.255.128 (/25) belongs to the subnet 192.168.48.128/25 (usable host range: 192.168.48.129 to 192.168.48.254).
Determining the valid subnet boundaries confirms which IP addresses are locally reachable.
2
Evaluate the configured default gateway address against the server's subnet.
The configured default gateway address 192.168.48.1 resides in the 192.168.48.0/25 subnet.
A host must have a default gateway within its own local IP subnet to resolve the gateway's MAC address via ARP and transmit off-subnet packets.
3
Identify the misconfiguration.
Because 192.168.48.1 and 192.168.48.150 are on separate IP subnets, the server considers the gateway off-link and cannot send traffic to it.
Host network stacks require the default gateway to be on the same broadcast domain and IP subnet.

Key Concept

Default Gateway Subnet Alignment
Estimated Time:1m 30s
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