5-2DHCP Address AllocationAllocation MechanismsDHCP supports three mechanisms for IP address allocation.z 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 5-2 Dynamic IP address allocation processAs shown in Figure 5-2, a DHCP client obtains an IP address from a DHCP server via four steps:2) The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.3) A DHCP server offers configuration parameters including an IP address to the client in aDHCP-OFFER message. The sending mode of the DHCP-OFFER message is determined by theflag field in the DHCP-DISCOVER message. Refer to DHCP Message Format for relatedinformation.4) If several DHCP servers send offers to the client, the client accepts the first received offer, andbroadcasts it in a DHCP-REQUEST message to formally request the IP address.5) All DHCP servers receive the DHCP-REQUEST message, but only the server from which theclient accepts the offered IP address responds. The server returns a DHCP-ACK message to theclient, confirming that the IP address has been allocated to the client, or a DHCP-NAK unicastmessage, denying the IP address allocation.
