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Difficulty: MediumIP Addressing and Network Configuration

A network technician manually configures network settings on a workstation attached to a corporate local subnet. The workstation is assigned an IP address of 10.0.15.50 with a subnet mask of 255.255.255.0 and a default gateway address of 10.0.16.1. The workstation can successfully ping neighboring hosts within the 10.0.15.0/24 network, but cannot reach any hosts on external subnets or the internet. Which of the following best explains the reason for this connectivity issue?

  1. The default gateway is configured on a different IP subnet than the host machine.Answer
  2. B
    The workstation failed to contact a DHCP server and automatically assigned itself an APIPA address.
  3. C
    The DNS server is configured to lease IP addresses rather than performing hostname resolution.
  4. D
    An unmanaged Layer 2 switch is being used instead of a router to connect local hosts on the 10.0.15.0/24 subnet.

Answer

The default gateway is configured on a different IP subnet than the host machine.
For a workstation to communicate with networks outside its local subnet, its configured default gateway must reside on the same IP subnet. With a /24 mask (255.255.255.0), the IP 10.0.15.50 is on the 10.0.15.0 network, while 10.0.16.1 is on the 10.0.16.0 network. Consequently, the workstation cannot send outbound traffic to the gateway router.

Step-by-Step Solution

1
Analyze the IP address configuration and subnet mask.
The host IP address 10.0.15.50 with a subnet mask of 255.255.255.0 (/24) places the host on the 10.0.15.0/24 subnet, covering IP addresses 10.0.15.1 through 10.0.15.254.
Determining the local subnet range allows verification of host and gateway compatibility.
2
Compare the default gateway address to the host subnet range.
The configured default gateway address 10.0.16.1 belongs to the 10.0.16.0/24 subnet, which is outside the local 10.0.15.0/24 broadcast domain.
For a host to forward out-of-subnet traffic, the default gateway must be a reachable Layer 3 router address located on the host's own local subnet.
3
Identify the cause of remote connectivity failure.
The host can reach local neighbors on 10.0.15.0/24 via ARP and Layer 2 switching, but it cannot deliver frames to 10.0.16.1 because that IP address is on another network segment.
A misconfigured default gateway on an improper subnet breaks all routing outside the local network.

Key Concept

Subnet Masking and Default Gateway Placement
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Topics

IP Addressing and Network Configuration

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