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Difficulty: HardIP Addressing and Network Configuration

A systems administrator is manually configuring a static IPv4 network interface on a new database server intended for the company subnet 10.50.12.0/2410.50.12.0/24. The administrator enters the following settings on the server:

- IP Address: 10.50.12.1510.50.12.15
- Subnet Mask: 255.255.255.128255.255.255.128
- Default Gateway: 10.50.12.25410.50.12.254

During initial testing, the server successfully pings local workstation addresses such as 10.50.12.5010.50.12.50, but cannot communicate with hosts on other VLANs or access the corporate internet gateway. Which of the following identifies the root cause of this connectivity failure?

  1. The configured subnet mask places the default gateway address on a different logical network segment, rendering it local-subnet unreachable.Answer
  2. B
    The server automatically assigned an APIPA address because the specified default gateway address did not respond to initial DHCP discover packets.
  3. C
    The DNS server configuration is missing, which prevents the host from resolving the default gateway IP address into a valid broadcast domain.
  4. D
    An unmanaged Layer 2 switch must be deployed to translate packet headers between hosts in the lower half and upper half of the IP range.

Answer

The configured subnet mask places the default gateway address on a different logical network segment, rendering it local-subnet unreachable.
The correct answer identifies that a subnet mask of 255.255.255.128 (/25) restricts the host's local network segment to the range 10.50.12.0 - 10.50.12.127. Because the default gateway IP is set to 10.50.12.254, the server evaluates the gateway address as being on a remote network. As a result, the server cannot ARP for the gateway MAC address locally, preventing any off-subnet traffic forwarding while allowing communication with local hosts in the 10.50.12.1 - 10.50.12.126 range.

Step-by-Step Solution

1
Analyze the configured IP address and subnet mask.
The IP address is 10.50.12.15 with a subnet mask of 255.255.255.128 (/25 prefix).
Determines the boundaries of the host's local IP subnet.
2
Calculate the usable host address range for the host's subnet.
The subnet range for 10.50.12.15/25 is 10.50.12.0 to 10.50.12.127 (usable host addresses: 10.50.12.1 to 10.50.12.126).
A 255.255.255.128 mask divides a /24 classful block into two 128-address subnets.
3
Compare the configured default gateway address against the local subnet range.
The configured gateway (10.50.12.254) belongs to the adjacent subnet (10.50.12.128/25 range 10.50.12.128 to 10.50.12.255).
A default gateway must reside within the same local IP subnet as the host interface to receive and forward off-subnet traffic.

Key Concept

Subnet Mask and Default Gateway Boundary Matching
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Topics

IP Addressing and Network Configuration

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