A system administrator is troubleshooting network reachability on a newly provisioned serv… — CompTIA Network+

A system administrator is troubleshooting network reachability on a newly provisioned server configured with IP address $172.16.14.78$ and subnet mask $255.255.255.192$ ( $/26$ ). The server can communicate with local host devices on its broadcast domain, but fails to send packets to remote subnets. The default gateway interface on the switch router is assigned $172.16.14.65/26$ . Upon reviewing the server's network configuration, the administrator notes that the default gateway entry is configured as $172.16.14.62$ . Which of the following accurately identifies the root cause of this routing issue?

The host IP address and configured default gateway reside on different subnets, preventing local Layer 2 frame delivery to the gateway.Answer
B
The configured default gateway address of $172.16.14.62$ is an unassigned subnet broadcast address and cannot process unicast packets.
C
An implicit deny statement at the end of the router's inbound Access Control List (ACL) is dropping outbound traffic from $172.16.14.78$ .
D
The server's default gateway port is configured for a protocol mismatch, blocking ICMP and IP forwarding across Layer 3 boundaries.

Answer

The host IP address and its configured default gateway belong to different subnets, causing Layer 2 resolution and IP routing to fail.

The server's IP address (

172.16.14.78/26

) is located within the

172.16.14.64/26

subnet (usable host range

172.16.14.65

172.16.14.126

). The misconfigured gateway address (

172.16.14.62

) belongs to the neighboring subnet (

172.16.14.0/26

). Because hosts can only send ARP requests for default gateways on their own local IP subnet, the server cannot establish Layer 2 communication with its gateway, preventing any outbound packet routing to remote networks.

Step-by-Step Solution

Determine the subnet boundaries for the server's assigned IP address and mask.

A mask of

255.255.255.192

corresponds to a

/26

prefix length with a block size of

64

(

256 - 192 = 64

). For IP

172.16.14.78

, the network boundary is

172.16.14.64/26

, giving a usable host range of

172.16.14.65

172.16.14.126

and broadcast address

172.16.14.127

Calculating the subnet range identifies which IP addresses are local to the host.

Evaluate the location of the configured default gateway address

172.16.14.62

The IP

172.16.14.62

falls into the range

172.16.14.0 - 172.16.14.63

(

172.16.14.0/26

), which is a separate IP subnet.

A host must reside on the same IP subnet as its default gateway to resolve its MAC address via ARP.

Identify the cause of remote unreachability.

Because

172.16.14.62

is on a foreign subnet relative to

172.16.14.78/26

, the server cannot send ARP requests locally to deliver frame payloads destined for off-subnet targets. Reconfiguring the gateway to

172.16.14.65

resolves the issue.

Layer 2 adjacency to the default gateway is mandatory for forwarding off-subnet traffic.

Key Concept

Default Gateway Subnet Mismatch Diagnosis

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

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