566 CHAPTER 32: DHCP OVERVIEWrelay agent. For information about the DHCP relay agent, refer to “Introduction toDHCP Relay Agent” on page 589.DHCP AddressAllocationAllocation Mechanisms DHCP supports three mechanisms for IP address allocation.■ Manual allocation: The network administrator assigns an IP address to a clientlike a WWW server, and DHCP conveys the assigned address to the client.■ Automatic allocation: DHCP assigns a permanent IP address to a client.■ Dynamic allocation: DHCP assigns an IP address to a client for a limited periodof time, which is called a lease. Most clients obtain their addresses in this way.Dynamic IP AddressAllocation ProcedureFigure 162 Dynamic IP address allocation processAs shown in the figure above, a DHCP client obtains an IP address from a DHCPserver via four steps:1 The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.2 A DHCP server offers configuration parameters such as an IP address to the clientin a DHCP-OFFER message. The sending mode of the DHCP-OFFER is determinedby the flag field in the DHCP-DISCOVER message. Refer to “DHCP MessageFormat” on page 567 for related information.3 If several DHCP servers send offers to the client, the client accepts the first receivedoffer, and broadcasts it in a DHCP-REQUEST message to formally request the IPaddress.4 All DHCP servers receive the DHCP-REQUEST message, but only the server towhich the client sent a formal request for the offered IP address returns aDHCP-ACK message to the client, confirming that the IP address has beenallocated to the client, or returns a DHCP-NAK unicast message, denying the IPaddress allocation.n ■ After the client receives the DHCP-ACK message, it will probe whether the IPaddress assigned by the server is in use by broadcasting gratuitous ARP. If theclient receives no response within specified time, the client can use this IP(1) DHCP-DISCOVER(2) DHCP-OFFER(3) DHCP-REQUEST(4) DHCP-ACKDHCP client DHCP server
