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Difficulty: MediumTroubleshooting Routing, Default Gateways, and ACLs

A network administrator is configuring a workstation in a newly provisioned subnet with the IPv4 address 172.16.45.50/25172.16.45.50/25. The host can successfully ping other devices on the 172.16.45.0/25172.16.45.0/25 local network, but cannot reach any external subnets or the internet. Executing `ipconfig` reveals the default gateway is configured as 172.16.45.130172.16.45.130. Which of the following is the root cause of the connectivity failure?

  1. The default gateway IP address resides outside the host's assigned IP subnet range.Answer
  2. B
    An implicit deny rule at the end of the router's inbound Access Control List is blocking ICMP traffic.
  3. C
    The host IP address is set to the subnet's broadcast address, preventing external packet routing.
  4. D
    The switchport connecting the workstation is assigned to an unrouted VLAN broadcast domain.

Answer

The default gateway IP address resides outside the host's assigned IP subnet range.
For a host to communicate with remote networks, its configured default gateway must reside on the exact same local IP subnet. A /25 prefix (subnet mask 255.255.255.128) provides usable host addresses from 172.16.45.1 to 172.16.45.126 with a broadcast address of 172.16.45.127. The configured default gateway IP address (172.16.45.130) belongs to the next block (172.16.45.128/25), making it unreachable via Layer 2 ARP requests.

Step-by-Step Solution

1
Calculate the subnet boundaries for 172.16.45.50/25172.16.45.50/25.
A /25/25 subnet mask (255.255.255.128255.255.255.128) divides the block into subnets of 128 addresses each: Subnet 1 spans 172.16.45.0172.16.45.0 to 172.16.45.127172.16.45.127 (usable host IPs: 172.16.45.1172.16.45.1–172.16.45.126172.16.45.126), and Subnet 2 spans 172.16.45.128172.16.45.128 to 172.16.45.255172.16.45.255.
Subnet bounds determine which IP addresses belong to the local broadcast domain versus remote networks requiring routing.
2
Evaluate the configured default gateway address (172.16.45.130172.16.45.130).
The IP 172.16.45.130172.16.45.130 lies within the second subnet (172.16.45.128/25172.16.45.128/25).
A host must have a default gateway configured on its local IP subnet to send frames directly to the gateway's MAC address via ARP.
3
Identify the root cause of the failure.
Because the default gateway IP is on a separate IP subnet, the workstation cannot resolve the gateway's Layer 2 address, causing all off-subnet traffic destined for external networks to fail.
Hosts cannot send traffic off-subnet without a valid, locally reachable default gateway.

Key Concept

Default Gateway Subnet Mismatch Diagnosis
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Topics

Troubleshooting Routing, Default Gateways, and ACLs

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