10 DHCP SERVER CONFIGURATIONIntroduction to DHCPServerThis section contains configuration introduction on DHCP Server.Usage of DHCP Server Generally, DHCP servers are used in the following networks to assign IP addresses:■ Large-sized networks, where manual configuration method bears heavy load andis difficult to manage the whole network in centralized way.■ Networks where the number of available IP addresses is less than that of the hosts.In this type of networks, IP addresses are not enough for all the hosts to obtain afixed IP address, and the number of on-line users is limited (such is the case in anISP network). In these networks, a great number of hosts must dynamically obtainIP addresses through DHCP.■ Networks where only a few hosts need fixed IP addresses and most hosts do notneed fixed IP addresses.DHCP Fundamentals Obtain IP address dynamicallyA DHCP client undergoes the following four phases to dynamically obtain an IPaddress from a DHCP server.■ Discover: The DHCP client tries to find a DHCP server by broadcasting aDHCP-DISCOVER packet.■ Offer: Each DHCP server that receives the DHCP-DISCOVER packet chooses anunassigned IP address from the address pool and sends a DHCP-OFFER packet(which carries the IP address and other configuration information) to the DHCPclient.■ Select: If more than one DHCP server sends a DHCP-OFFER packet to the DHCPclient, the DHCP client only accepts the DHCP-OFFER packet that first arrives, andthen broadcasts a DHCP-REQUEST packet containing the assigned IP addresscarried in the DHCP-OFFER packet.■ 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 assignment ofthe IP address to the client. When the client receives the DHCP-ACK packet, itbroadcasts an ARP packet with the assigned IP address as the destination addressto detect the assigned IP address, and uses the IP address only if it does not receiveany response within a specified period.The IP addresses offered by other DHCP servers (if any) are not used by the DHCPclient and are still available to other clients.
