A network administrator is troubleshooting an issue where a workstation configured with IP… — CompTIA Network+

A network administrator is troubleshooting an issue where a workstation configured with IP address $10.200.48.50$ and subnet mask $255.255.240.0$ (/20) can communicate with other hosts on its local network but cannot access external destinations. Running `traceroute 198.51.100.25` fails immediately on the first hop. Reviewing the workstation's network settings reveals that the default gateway is configured as $10.200.64.1$ . Which of the following is the root cause of the connectivity issue?

The default gateway address resides on a different subnet than the host IP address.Answer
B
An access control list on the local switch dropped the ICMP packet due to an implicit deny rule.
C
The default gateway IP address is invalid because it is the broadcast address for the host's subnet.
D
The host and default gateway are separated by an unrouted VLAN broadcast domain boundary.

Answer

The default gateway address resides on a different subnet than the host IP address.

Subnet mask

255.255.240.0

(/20) dictates that subnets increment by 16 in the third octet (

256 - 240 = 16

). The host network is

10.200.48.0/20

, spanning addresses

10.200.48.0

through

10.200.63.255

. The configured gateway address

10.200.64.1

belongs to the

10.200.64.0/20

network block. Because a host must send Layer 2 ARP requests to discover its default gateway, the gateway must reside on the same IP subnet as the host interface.

Step-by-Step Solution

Calculate the network range for IP 10.200.48.50 with subnet mask 255.255.240.0 (/20).

A /20 subnet mask yields a block size of 16 in the third octet (

256 - 240 = 16

). Network address: 10.200.48.0. Usable range: 10.200.48.1 to 10.200.63.254. Broadcast address: 10.200.63.255.

Defining the subnet boundaries determines which destinations the host considers local.

Evaluate the configured default gateway IP address relative to the local subnet.

The configured gateway IP address 10.200.64.1 falls into the next subnet block (10.200.64.0/20).

A host can only ARP directly for a default gateway that shares its local IP subnet.

Determine the root cause of the destination unreachability.

Because 10.200.64.1 is off-subnet, the host cannot ARP for its default gateway to forward off-subnet traffic.

Reconfiguring the workstation's default gateway to a valid IP within 10.200.48.0/20 (such as 10.200.48.1) restores external connectivity.

Key Concept

Default Gateway Subnet Alignment

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