When a client receives an IP it also receive a lease duration from the DHCP server, when the client holds IP for half of the assigned lease duration (If the lease duration is 8 hrs and the client completed 4 hrs) then the client sends request packet which then receive ack packet for renewing it for next 8 hrs. If the client was unable to communicate with the DHCP server for the complete 8 hrs then the IP is flushed and the client again performs DORA process to get a new IP.
