A network administrator is investigating connectivity complaints from users on a newly dep… — CompTIA Network+

A network administrator is investigating connectivity complaints from users on a newly deployed subnet ( $10.20.50.0/24$ ). An affected workstation displays an IP address of $169.254.112.45$ and a subnet mask of $255.255.0.0$ . The organization's central DHCP server is located on a separate administrative segment ( $10.20.10.10$ ). Which of the following issues could cause the client to receive this address? (Select TWO.)

The local router interface serving as the default gateway for $10.20.50.0/24$ is missing a DHCP relay agent (ip helper-address) configuration.Answer
The DHCP scope designated for the $10.20.50.0/24$ subnet on the central server has exhausted all available IP addresses.Answer
C
The host network interface card and switch port are operating with a speed and duplex mismatch.
D
The host system rejected the lease assignment because its static default gateway belonged to a different IP subnet.

Answer

The client receiving an Automatic Private IP Addressing (APIPA) address indicates a failure to communicate with or obtain an IP lease from a DHCP server. The two valid root causes are: 1) missing DHCP relay (IP helper) configuration on the local default gateway router interface, which prevents broadcast discovery frames from reaching the remote DHCP server, and 2) scope exhaustion on the DHCP server for that specific subnetwork pool.

An APIPA address (169.254.0.1 through 169.254.255.254) indicates that the client attempted to obtain an IP lease via DHCP but received no response. When a DHCP server resides on a different IP subnet than the client, broadcast DHCPDISCOVER packets require a Layer 3 relay agent (such as ip helper-address on network routers) to unicast the request to the central DHCP server. Without this relay, requests are dropped by the default gateway. Alternatively, if the relay is functioning but the DHCP server has run out of available addresses in its scope pool for that subnet, no lease can be granted, resulting in APIPA self-assignment.

Step-by-Step Solution

Analyze the reported IP address signature.

The host IPv4 address

169.254.112.45

with mask

255.255.0.0

is an APIPA address assigned when a DHCP client gets no reply to DHCPDISCOVER broadcasts.

Identifying APIPA establishes that the core problem is DHCP request/response failure rather than an inline routing or Layer 1 hardware failure.

Evaluate subnetwork boundaries between client and DHCP server.

The client is on

10.20.50.0/24

while the DHCP server is on

10.20.10.10

. Broadcasts do not traverse routers without a relay agent.

If the local router interface lacks an ip helper-address directive, DHCPDISCOVER packets are dropped at the Layer 3 boundary.

Evaluate server-side scope availability.

If the relay agent is active but the DHCP server scope has zero available leases, the server cannot offer an address.

Scope exhaustion produces the identical APIPA symptom on client machines.

Key Concept

APIPA Diagnosis and DHCP Relay Configuration

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