3Com Switch 8800 Advanced Software V5 Configuration Guide

722 CHAPTER 50: DHCP SERVER CONFIGURATION
At the very beginning, subnetworks inherit network parameters and clients inherit
subnetwork parameters. Therefore, common parameters, for example the domain
name, should be configured at the highest (network or subnetwork) level of the
tree.
After establishment of the inheritance relationship, the new configuration at the
higher level of the tree will be:
■ Inherited if the lower level has no such configuration, or
■ Overridden if the lower level has such configuration.
n
The IP address lease does not have any inheritance.
The DHCP server observes the following principles to select an address pool to
assign IP addresses to clients:
1 If there is an address pool where IP addresses are statically bound to the MAC
addresses or IDs of clients, the DHCP server will select this address pool and assign
statically bound IP addresses to clients. For the configuration of this address pool,
refer to
“Configure manual address allocation” on page 724.
2 Otherwise, the DHCP server will select the smallest address pool that contains the
IP address of the interface receiving DHCP requests, regardless of the mask. If no IP
address is available in the smallest address pool, the DHCP server will fail to assign
addresses to clients because it will not assign those in the father address pool to
clients. For the configuration of the smallest address pool, refer to
“Configure
dynamic address allocation” on page 725.
For example, two address pools are configured on the DHCP server. The ranges of
IP addresses that can be dynamically assigned are 1.1.1.0/24 and 1.1.1.0/25
respectively. If the IP address of the interface receiving DHCP requests is
1.1.1.1/25, the DHCP server will select IP addresses for clients from the 1.1.1.0/25
address pool. If no IP address is available in the 1.1.1.0/25 address pool, the DHCP
server will fail to assign addresses to clients. If the IP address of the interface
receiving DHCP requests is 1.1.1.130/25, the DHCP server will select IP addresses
for clients from the 1.1.1.0/24 address pool.
n
Keep the IP addresses for dynamic allocation within the subnet where the
interface of the DHCP server resides to avoid wrong IP address allocation.
IP Address Allocation
Sequence
A DHCP server assigns an IP address to a client according to the following
sequence:
1 The IP address manually bound to the client’s MAC address or ID
2 The IP address that was ever assigned to the client
3 The IP address designated by the Option 50 field in a DHCP-DISCOVER message
4 The first IP address found in the DHCP address pool
5 The IP address that was a conflict or passed its lease duration
If no IP address is assignable, the server will not respond.