If a client is configured to receive its IPv4 settings dynamically and wants to join the network, it requests addressing values from the DHCPv4 server. The client transmits a DHCPDISCOVER message on its local network when it boots or senses an active network connection. Because the client has no way of knowing the subnet to which it belongs, the DHCPDISCOVER message is an IPv4 broadcast (destination IPv4 address of 255.255.255.255). The client does not have a configured IPv4 address yet, so the source IPv4 address of 0.0.0.0 is used.
As shown in Figure 1, the client IPv4 address (CIADDR), default gateway address (GIADDR), and subnet mask are all marked to indicate that the address 0.0.0.0 is used.
Note: Unknown information is sent as 0.0.0.0.
When the DHCPv4 server receives the DHCPDISCOVER message, it responds with a DHCPOFFER message. This message contains initial configuration information for the client, including the IPv4 address that the server offers, the subnet mask, the lease duration, and the IPv4 address of the DHCPv4 server making the offer.
The DHCPOFFER message can be configured to include other information, such as the lease renewal time and DNS address.
As shown in Figure 2, the DHCP server responds to the DHCPDISCOVER by assigning values to the CIADDR and subnet mask. The frame is constructed using the client hardware address (CHADDR) and sent to the requesting client.
The client and server send acknowledgment messages, and the process is complete.