Client Conflict Detection
Client computers running Windows Server 2003, Windows XP, Windows 2000, Windows NT 4.0, Windows Millennium Edition, and Windows 98 automatically check to determine if an IP address is already in use before using it.
After the DHCP client receives a lease from the DHCP server, the client sends an Address Resolution Protocol (ARP) request to the address that it has been assigned. If a reply to the ARP request is received, the client has detected a conflict and sends a DHCPDecline message to the DHCP server. The DHCP server attaches a BAD_ADDRESS value to the IP address in the scope for the length of the lease. The client then begins the lease process again, and is offered the next available address in the scope.
Note
In general, use server conflict detection only as a troubleshooting aid when you suspect that duplicate IP addresses are in use on your network. Each additional conflict detection attempt adds to the time needed to negotiate leases for DHCP clients.