Final answer:
A DHCP client sends a DHCPDISCOVER broadcast message when its IP address lease has expired to obtain a new IP configuration from a DHCP server, since it may not have a valid IP address for direct communication.
Step-by-step explanation:
When a DHCP client's IP address lease has expired, it sends a DHCPDISCOVER broadcast message. This message is a broadcast because the client may no longer have a valid IP address, and as such, cannot communicate directly with the DHCP server through unicast. The DHCPDISCOVER message is used by the client to initiate a conversation with a DHCP server to obtain the necessary IP configuration settings, including a new IP address.
The other messages, such as the DHCPREQUEST, may be used in different stages of the DHCP IP address allocation process. For example, a DHCPREQUEST broadcast message is sent after receiving an offer from a DHCP server in the form of a DHCPOFFER message, as the client attempts to request that specific IP address. Both the discovery and request processes are vital for network communication among devices that rely on DHCP for IP address allocation.