3Com 5500-EI PWR Install Manual page 582

Types of address pool
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.
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 address of the natural network segment, the branches hold the subnet IP addresses, and the
leaves holds 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 natural 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 segment or some subnets (such as domain name),
you just need to configure them on the network segment or the corresponding subnets. The following is
the details of configuration inheritance.
1) A newly created child address pool inherits the configurations of its parent address pool.
2) For an existing parent-child address pool pair, when you performs a new configuration on the
parent address pool:
The child address pool inherits the new configuration if there is no corresponding configuration on
the child address pool.
The child address pool does not inherit the new configuration if there is already a corresponding
configuration on the child address pool.
The IP address lease does not enjoy the inheritance attribute.
Principles of address pool selection
The DHCP server observes the following principles to select an address pool to assign an IP address to
a client:
1) If the receiving interface works in the global address pool mode, the DHCP server assigns an IP
address from the global address pool to the DHCP client.
2) If the receiving interface works in the interface address pool mode, the DHCP server assigns an IP
address from the interface address pool to the DHCP client directly connected to the interface. If
there is no available IP address in the interface address pool, the DHCP server selects an IP
address from the global address pool that contains the interface address pool's network segment
for the client.
The DHCP server assigns an IP address to the client in the following order from an interface address
pool or a global address pool:
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