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Difficulty: HardTroubleshooting Routing, Default Gateways, and ACLs

A network systems engineer is troubleshooting an issue where a newly provisioned Linux database server cannot communicate with remote clients or download updates from an off-site repository at 172.16.50.10172.16.50.10. The server interface configuration and routing table display the following CLI output:

$ ip addr show dev eth0
inet 192.168.45.200/27 brd 192.168.45.223 scope global eth0

$ ip route show
default via 192.168.45.225 dev eth0
192.168.45.192/27 dev eth0 proto kernel scope link src 192.168.45.200

$ ping -c 2 172.16.50.10
connect: Network is unreachable

Based on the diagnostic output, which of the following identifies the root cause of the connectivity failure?

  1. The configured default gateway address resides outside the host's IPv4 subnet boundaries.Answer
  2. B
    The server's IP address is set to the network broadcast address of the local subnet.
  3. C
    The default gateway router interface is assigned to an incorrect VLAN broadcast domain on the switch.
  4. D
    An outbound Access Control List on the router is dropping packets due to an implicit deny rule.

Answer

The configured default gateway address resides outside the host's IPv4 subnet boundaries.
The CIDR prefix /27 establishes subnets in increments of 32 host addresses. For the network starting at 192.168.45.192/27192.168.45.192/27, the usable host addresses span from 192.168.45.193192.168.45.193 to 192.168.45.222192.168.45.222, and the broadcast address is 192.168.45.223192.168.45.223. The default gateway address of 192.168.45.225192.168.45.225 belongs to the next network segment (192.168.45.224/27192.168.45.224/27). Because the default gateway is not on the same local subnet as the server interface, the operating system cannot ARP for the gateway or route packets out of the interface, generating the 'Network is unreachable' error.

Step-by-Step Solution

1
Calculate the subnet boundaries from the CIDR prefix
For prefix /27, the block size is 232−27=322^{32-27} = 32. The network address is 192.168.45.192192.168.45.192, giving a host address range of 192.168.45.193192.168.45.193 through 192.168.45.222192.168.45.222, with broadcast 192.168.45.223192.168.45.223.
Determining the valid usable IP address range is necessary to check if the host and gateway reside on the same Layer 3 segment.
2
Analyze the configured host IP and default gateway IP
Host IP 192.168.45.200192.168.45.200 falls within 192.168.45.192/27192.168.45.192/27. The default gateway 192.168.45.225192.168.45.225 falls in the adjacent subnet 192.168.45.224/27192.168.45.224/27.
A host can only directly communicate via Layer 2 ARP with a default gateway that shares its local IP subnet.
3
Evaluate the kernel error message
The OS kernel rejects sending off-subnet packets because the target default gateway cannot be resolved on the directly attached link, raising 'Network is unreachable'.
Confirms the failure originates at local Layer 3 configuration rather than network ACLs or switch VLANs.

Key Concept

Default Gateway Subnet Alignment
Estimated Time:2m 0s
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Topics

Troubleshooting Routing, Default Gateways, and ACLs

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