After obtaining an IPv4 address from a DHCP server, how does a DHCP client ensure that no other computer is already using that address on the local network?

Introduction / Context:
IP address conflicts can cause serious connectivity problems on a local area network. DHCP includes mechanisms to minimize the risk of two hosts using the same IP address. One of these mechanisms involves the use of ARP messages at the data link layer to verify that an address is not already in use before a client begins normal communication with that address.

Given Data / Assumptions:

• A client has just received an IPv4 address and related configuration from a DHCP server.
• The address is within a shared broadcast domain, such as an Ethernet LAN.
• The client must check for conflicts before fully using this address.
• We are considering standard behavior described in DHCP and ARP practices.

Concept / Approach:
A gratuitous ARP is an ARP request or reply that announces a host's own IP-to-MAC mapping, often sent without being explicitly requested by another device. When a client sends a gratuitous ARP for its newly assigned IP, it both claims the address and checks for duplicates. If another device is already using that IP, it may respond, alerting the client to a conflict. This technique goes beyond simply pinging the address because ARP operates at the link layer, where conflicts are first detected.

Step-by-Step Solution:
1. The DHCP client receives an address and configuration from the DHCP server.2. Before fully adopting the address, the client sends a gratuitous ARP on the local network that includes its own IP and MAC.3. This ARP is broadcast to all devices in the LAN segment.4. If no other device responds with a conflicting ARP, the client assumes that the address is unique and safe to use.5. If a response indicating a duplicate appears, the client or server can take steps to resolve the conflict, such as selecting a different address.

Verification / Alternative check:
Packet captures on a DHCP-enabled network show clients sending ARP probes or gratuitous ARPs after address assignment. Observing this traffic confirms that the client actively tests for conflicts as part of its startup behavior. Some systems also log IP conflict messages if a duplicate is detected, which further demonstrates the importance of gratuitous ARP.

Why Other Options Are Wrong:
Option A mentions acknowledging a TCP segment, which is unrelated to address conflict detection at startup. Option B suggests pinging the address, but ICMP echo is less reliable than ARP for detecting conflicts and is not the standard method used by DHCP clients. Option C refers to Proxy ARP, where a router answers ARP requests on behalf of remote hosts; this is not how a client checks its own address.

Common Pitfalls:
A common misconception is that DHCP alone prevents conflicts through careful lease management. In reality, ARP-based checks provide an additional safety net. Another pitfall is not recognizing gratuitous ARP packets in network traces and missing evidence of conflict detection mechanisms.

Final Answer:
Broadcast a gratuitous ARP announcing its own IP-to-MAC mapping and checking for conflicts.