Server Conflict Detection
The DHCP server detects conflicts by pinging an IP address before offering that address to clients. If the ping is successful (a response is received from a computer), a conflict is registered and that address is not offered to clients requesting a lease from the server. The DHCP server pings only addresses that have not been successfully and previously leased. If a client receives a lease on an IP address that it already had or is requesting a renewal, the DHCP server does not send a ping.
If conflict detection is enabled, an administrator-defined number of pings are sent. The server waits 1 second for a reply. Because the time required for a client to obtain a lease is equal to the number of pings selected, choose this value carefully as it directly impacts the overall performance of the server. In general, one ping should be sufficient.
A DHCP server receiving a reply to any of the pings (meaning there is a conflict) attaches a BAD_ADDRESS value to that IP address in the scope, and will try to lease the next available address. If the duplicate address is removed from the network, the BAD_ADDRESS value attached to the IP address can be deleted from the scope's list of active leases, and the address returned to the pool. Addresses are marked as BAD_ADDRESS for the length of the lease for which the scope is configured.
If your network includes legacy DHCP clients, enable conflict detection on the DHCP server. By default, the DHCP service does not perform any conflict detection. In general, conflict detection should be used only as a troubleshooting aid when you suspect there are duplicate IP addresses in use on your network. The reason for this is that, for each additional conflict detection attempt that the DHCP service performs, additional seconds are added to time needed to negotiate leases for DHCP clients.