When the system running the DHCP client shuts down normally, the dhcpagent daemon writes the current configuration information to the file /etc/dhcp/interface.dhc. The lease is dropped rather than released, so the DHCP server does not know that the IP address is not in active use.
If the lease is still valid when the system reboots, the DHCP client sends an abbreviated request to use the same IP address and network configuration information it had used before the system rebooted. If the DHCP server permits this, the client can use the information that it wrote to disk when the system shut down. If the server does not permit the client to use the information, the client initiates the DHCP protocol sequence described previously and obtains new network configuration information.