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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator is setting up a new server on a subnet designed for database hosts: 172.16.10.128/26172.16.10.128/26. The default gateway router interface for this subnet is assigned IP address 172.16.10.129/26172.16.10.129/26. The administrator manually configures the server with IP address 172.16.10.66172.16.10.66 and subnet mask 255.255.255.192255.255.255.192 (/26/26). Upon testing, the server cannot ping its default gateway or reach any external networks. Which of the following best identifies the root cause of this connectivity failure?

  1. The host IP address assigned to the server resides on a different IP subnet than the default gateway.Answer
  2. B
    The server failed to receive a DHCP lease and automatically assigned itself an Automatic Private IP Addressing (APIPA) address.
  3. C
    The local router switchport is missing an IP helper address configuration to forward DHCP broadcast requests.
  4. D
    The IP address assigned to the default gateway is the reserved network broadcast address for the subnet.

Answer

The host IP address assigned to the server resides on a different IP subnet than the default gateway.
With a /26/26 subnet mask (255.255.255.192255.255.255.192), subnets increment in blocks of 64 addresses. The default gateway IP 172.16.10.129172.16.10.129 resides in the 172.16.10.128/26172.16.10.128/26 subnet (usable host range 172.16.10.129−172.16.10.190172.16.10.129 - 172.16.10.190). However, the static host IP 172.16.10.66172.16.10.66 resides in the adjacent 172.16.10.64/26172.16.10.64/26 subnet (usable host range 172.16.10.65−172.16.10.126172.16.10.65 - 172.16.10.126). Because the host and default gateway are on different logical IP subnets, the host cannot complete ARP resolution for the default gateway IP address, resulting in total unreachability.

Step-by-Step Solution

1
Analyze the subnet boundaries for a /26 subnet mask (255.255.255.192).
A /26 mask creates subnets with block sizes of 64 addresses: 172.16.10.0/26 (0-63), 172.16.10.64/26 (64-127), and 172.16.10.128/26 (128-191).
Determining exact subnet ranges identifies which addresses belong to the same local broadcast domain.
2
Map the configured server IP address to its corresponding subnet range.
The server IP 172.16.10.66 falls into the 172.16.10.64/26 subnet (usable range 172.16.10.65 to 172.16.10.126).
Confirms the host's actual local subnet.
3
Compare host IP subnet to default gateway IP subnet.
The gateway IP 172.16.10.129 resides in the 172.16.10.128/26 subnet (usable range 172.16.10.129 to 172.16.10.190).
Because the host and gateway are on separate subnets, the host cannot resolve the gateway's MAC address via local ARP requests, resulting in host unreachability.

Key Concept

Default Gateway Subnet Alignment and CIDR Boundary Troubleshooting
Estimated Time:1m 30s
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Topics

Troubleshooting IP Addressing and DHCP Services

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