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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator is investigating a connectivity issue on a newly deployed workstation on the finance floor. The user reports being able to communicate with other devices on the local subnet, but cannot access external services or the internet. The administrator runs `ipconfig /all` on the client machine and observes the following output:

IPv4 Address . . . . . . . . . . . : 10.45.20.11510.45.20.115
Subnet Mask . . . . . . . . . . . . : 255.255.255.0255.255.255.0
Default Gateway . . . . . . . . . . : 10.45.21.110.45.21.1
DHCP Server . . . . . . . . . . . . : 10.45.10.1510.45.10.15

Which of the following identifies the root cause of the workstation's external connectivity failure?

  1. The configured default gateway address resides on a different IP subnet than the host IP address.Answer
  2. B
    The host failed to reach the DHCP server and automatically assigned itself an APIPA address.
  3. C
    The router's DHCP relay agent failed to forward the DHCP lease requests across subnets.
  4. D
    The DHCP pool scope has exhausted all available host addresses on the 10.45.20.0/2410.45.20.0/24 network.

Answer

The configured default gateway address resides on a different IP subnet than the host IP address.
The host IP address (10.45.20.11510.45.20.115) with mask 255.255.255.0255.255.255.0 places the device on the 10.45.20.0/2410.45.20.0/24 network. The default gateway (10.45.21.110.45.21.1) resides on the 10.45.21.0/2410.45.21.0/24 network. A host cannot route packets outside its local segment if its default gateway is on a different subnet, resulting in local-only network connectivity.

Step-by-Step Solution

1
Calculate the workstation's local network address range
Host IP 10.45.20.11510.45.20.115 with Subnet Mask 255.255.255.0255.255.255.0 means the local subnet is 10.45.20.0/2410.45.20.0/24 (usable host IPs: 10.45.20.110.45.20.1 to 10.45.20.25410.45.20.254).
Determines the boundaries of the local broadcast domain.
2
Evaluate the default gateway IP address against the local subnet boundaries
The configured gateway 10.45.21.110.45.21.1 falls outside the 10.45.20.0/2410.45.20.0/24 subnet.
A default gateway must be a Layer 3 interface directly reachable on the local IP subnet segment.
3
Correlate symptoms with the configuration mismatch
Local traffic works because ARP resolves local IPs directly; remote traffic fails because the host cannot reach its default gateway on the local segment.
Explains why intra-subnet connectivity succeeds while inter-subnet/internet traffic fails.

Key Concept

Default Gateway Subnet Alignment
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Topics

Troubleshooting IP Addressing and DHCP Services

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