3Com 10014299 manual Occasions in which Dhcp server is applied, Following figure

334CHAPTER 21: CONFIGURING IP APPLICATION

DHCP server can provide three types of IP address allocation policies according to the different requirements:

■Allocate addresses manually: Administrators configure special IP addresses for a small number of particular hosts such as the service server WWW.

■Allocate addresses automatically: Allocate permanent IP addresses for some hosts connected to the network for the first time and the addresses will be allocated to the hosts for a long period of time.

■Allocate addresses dynamically: Allocate some addresses to client hosts by means of “leasing”. In this case, the expiry date is limited and clients should re-apply for new addresses upon the expiry. Most of clients are offered such dynamic addresses.

Occasions in which DHCP server is applied

DHCP service is usually used to implement the allocation of IP addresses in the following occasions:

■Network scale is relatively large, manual configuration will consume an enormous working volume and at the same time, it is difficult to centralize the management of the overall network.

■Hosts on the network are more than the IP addresses supported by this network. That is, a fixed IP address cannot be allocated to each host. For example, Internet accessing operators are in this situation. Numerous users must dynamically obtain their own IP addresses through the DHCP service and the number of simultaneous users is limited to a certain degree.

■There are only a few hosts with their own fixed IP addresses on the network (for example, various server hosts need fixed IP addresses) while most hosts have no requirement for fixed IP addresses.

Fundamentals of DHCP Normally, a typical DHCP application network comprises of a DHCP server and Server numerous client computers such as PCs and portable computers, as shown in the

following figure:

Figure 124 Typical networking application of DHCP server

LAN

DHCP server

DHCP clients

To obtain a legal dynamic IP address (the processes of obtaining an auto-allocated address and a manually allocated address are similar), a DHCP client should interact different information with the server in different stages. Normally, there are three types of modes: