1-2Figure 1-1 Typical DHCP applicationDHCP IP Address AssignmentIP Address Assignment PolicyCurrently, DHCP provides the following three IP address assignment policies to meet the requirementsof different clients:z Manual assignment. The administrator configures static IP-to-MAC bindings for some specialclients, such as a WWW server. Then the DHCP server assigns these fixed IP addresses to theclients.z Automatic assignment. The DHCP server assigns IP addresses to DHCP clients. The IPaddresses will be occupied by the DHCP clients permanently.z Dynamic assignment. The DHCP server assigns IP addresses to DHCP clients for predeterminedperiod of time. In this case, a DHCP client must apply for an IP address again at the expiration ofthe period. This policy applies to most clients.Obtaining IP Addresses DynamicallyA DHCP client undergoes the following four phases to dynamically obtain an IP address from a DHCPserver:1) Discover: In this phase, the DHCP client tries to find a DHCP server by broadcasting aDHCP-DISCOVER packet.2) Offer: In this phase, the DHCP server offers an IP address. After the DHCP server receives theDHCP-DISCOVER packet from the DHCP client, it chooses an unassigned IP address from theaddress pool according to the priority order of IP address assignment and then sends the IPaddress and other configuration information together in a DHCP-OFFER packet to the DHCPclient. The sending mode is decided by the flag filed in the DHCP-DISCOVER packet, refer tosection DHCP Packet Format for details.3) Select: In this phase, the DHCP client selects an IP address. If more than one DHCP server sendsDHCP-OFFER packets to the DHCP client, the DHCP client only accepts the DHCP-OFFERpacket that first arrives, and then broadcasts a DHCP-REQUEST packet containing the assignedIP address carried in the DHCP-OFFER packet.4) Acknowledge: In this phase, the DHCP servers acknowledge the IP address. Upon receiving theDHCP-REQUEST packet, only the selected DHCP server returns a DHCP-ACK packet to theDHCP client to confirm the assignment of the IP address to the client, or returns a DHCP-NAKpacket to refuse the assignment of the IP address to the client. When the client receives the
