HP 4800G Series Configuration Manual page 270

Common address pool structure
In response to a client's request, the DHCP server selects an idle IP address from an address pool and
sends it together with other parameters such as lease and DNS server address to the client.
The common address pool database is organized as a tree. The root of the tree is the address pool for
natural networks, branches are address pools for subnets, and leaves are addresses statically bound to
clients. For the same level address pools, a previously configured pool has a higher selection priority
than a new one.
At the very beginning, subnetworks inherit network parameters and clients inherit subnetwork
parameters. Therefore, common parameters, for example a DNS server address, 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 (father) of
the tree will be:
Inherited if the lower level (child) has no such configuration, or
Overridden if the lower level (child) has such configuration.
The extended address pool database is not organized as a tree.
The IP address lease does not enjoy the inheritance attribute.
Principles for selecting an address pool
The DHCP server observes the following principles to select an address pool when assigning an IP
address to a client:
1) If the receiving interface has an extended address pool referenced, the DHCP server will assign an
IP address from this address pool. If no IP address is available in the address pool, the DHCP
server will fail to assign an address to the client. For the configuration of such an address pool,
refer to section Configuring Dynamic Address Allocation for an Extended Address
2) If there is an address pool where an IP address is statically bound to the MAC address or ID of the
client, the DHCP server will select this address pool and assign the statically bound IP address to
the client. For the configuration of this address pool, refer to section
allocation.
3) Otherwise, the DHCP server will select the smallest common address pool that contains the IP
address of the receiving interface (if the client and the server reside on the same network segment),
or the smallest common address pool that contains the IP address specified in the giaddr field of
the client's request (if a DHCP relay agent is in-between). If no IP address is available in the
address pool, the DHCP server will fail to assign an address to the client because it cannot assign
an IP address from the father address pool to the client. For the configuration of such address pool,
refer to section Configuring dynamic address
For example, two common address pools, 1.1.1.0/24 and 1.1.1.0/25, are configured on the DHCP
server. 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 address pool 1.1.1.0/25. If no IP address is available in the address
pool, the DHCP server will fail to assign addresses to clients. If the IP address of the interface receiving
allocation.
2-2
Pool.
Configuring manual address