Operation Manual – DHCPH3C S7500 Series Ethernet Switches Chapter 1 DHCP Overview1-21.2 DHCP IP Address Assignment1.2.1 IP Address Assignment PolicyCurrently, DHCP provides the following three IP address assignment policies to meetthe requirements of different clients:z Manual assignment. The administrator statically binds IP addresses to fewclients with special uses (such as WWW server). Then the DHCP server assignsthese fixed IP addresses to the clients.z Automatic assignment. The DHCP server assigns IP addresses to DHCP clients.The IP addresses will be occupied by the DHCP clients permanently.z Dynamic assignment. The DHCP server assigns IP addresses to DHCP clientsfor predetermined period of time. In this case, a DHCP client must apply for an IPaddress at the expiration of the period. This policy applies to most clients.1.2.2 Obtaining IP Addresses DynamicallyA DHCP client undergoes the following four phases to dynamically obtain an IPaddress from a DHCP server:1) Discover: In this phase, the DHCP client tries to find a DHCP server bybroadcasting a DHCP-DISCOVER packet.2) Offer: In this phase, the DHCP server offers an IP address. Each DHCP serverthat receives the DHCP-DISCOVER packet chooses an unassigned IP addressfrom the address pool based on the IP address assignment policy and thensends a DHCP-OFFER packet (which carries the IP address and otherconfiguration information) to the DHCP client. The transmission mode dependson the flag field in the DHCP-DISCOVER packet. For details, see section DHCPPacket Format.3) Select: In this phase, the DHCP client selects an IP address. If more than oneDHCP server sends DHCP-OFFER packets to the DHCP client, the DHCP clientonly accepts the DHCP-OFFER packet that first arrives, and then broadcasts aDHCP-REQUEST packet containing the assigned IP address carried in theDHCP-OFFER packet.4) Acknowledge: Upon receiving the DHCP-REQUEST packet, the DHCP serverreturns a DHCP-ACK packet to the DHCP client to confirm the assignment of theIP address to the client, or returns a DHCP-NAK packet to refuse the assignmentof the IP address to the client. When the client receives the DHCP-ACK packet, itbroadcasts an ARP packet with the assigned IP address as the destinationaddress to detect the assigned IP address, and uses the IP address only if itdoes not receive any response within a specified period.
