3Com Switch 7754, Configuration Manual

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57
DHCP S
ERVER
C
ONFIGURATION
Introduction to 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
and is difficult to manage the whole network in a 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 a fixed IP address, and the number of on-line users is limited (such is the
case in an ISP network). In these networks, a great number of hosts must
dynamically obtain IP addresses through DHCP.
■
Networks where only a few hosts need fixed IP addresses and most hosts do
not need fixed IP addresses.
DHCP Address Pool A DHCP address pool holds the IP addresses to be assigned to DHCP clients. When
a DHCP server receives a DHCP request from a DHCP client, it selects an address
pool depending on the configuration, picks an IP address from the pool and sends
the IP address and other related parameters (such as the IP address of the DNS
server, and the lease time of the IP address) to the DHCP client.
Types of address pools
The address pools of a DHCP server fall into two types: global address pool and
interface address pool.
■
A global address pool is created by executing the dhcp server ip-pool
command in system view. It is valid on the current device.
■
If an interface is configured with a valid unicast IP address, you can create an
interface-based address pool for the interface by executing the
dhcp select
interface command in interface view. The IP addresses an interface address
pool holds belong to the network segment the interface resides in and are
available to the interface only.
The structure of an address pool
The address pools of a DHCP server are hierarchically organized in a tree-like
structure. The root holds the IP addresses of the network segment, the branches
hold the subnet IP addresses, and the leaves hold the IP addresses that are
manually bound to specific clients. The address pools that are of the same level are
sorted by their configuration precedence order. Such a structure enables
configurations to be inherited. That is, the configurations of the network segment
can be inherited by its subnets, whose configurations in turn can be inherited by
their client address. So, for the parameters that are common to the whole network