1-2z Manual allocation: The network administrator assigns an IP address to a client like a WWW server,and DHCP conveys the assigned address to the client.z Automatic allocation: DHCP assigns a permanent IP address to a client.z Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which iscalled a lease. Most DHCP clients obtain their addresses in this way.Dynamic IP Address Allocation ProcessFigure 1-2 Dynamic IP address allocation processAs shown in Figure 1-2, a DHCP client obtains an IP address from a DHCP server 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 client in aDHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field inthe DHCP-DISCOVER message.3) If several DHCP servers send offers to the client, the client accepts the first received offer, andbroadcasts it in a DHCP-REQUEST message to request the IP address formally.4) All DHCP servers receive the DHCP-REQUEST message, but only the server from which the clientaccepts the offered IP address returns a DHCP-ACK message to the client, confirming that the IPaddress has been allocated to the client, or a DHCP-NAK unicast message, denying the IPaddress allocation.z After the client receives the DHCP-ACK message, it will probe whether the IP address assigned bythe server is in use by broadcasting a gratuitous ARP packet. If the client receives no responsewithin the specified time, the client can use this IP address. Otherwise, the client sends aDHCP-DECLINE message to the server and requests an IP address again.z IP addresses offered by other DHCP servers are still assignable to other clients.
