DHCP address pool

DHCP address pools include common and extended address pools.

• Common address pool: Supports both static binding and dynamic allocation.

• Extended address pool: Supports only dynamic allocation.

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, subnets inherit network parameters and clients inherit subnet parameters. Therefore, common parameters, for example a DNS server address, should be configured at the highest (network or subnet) level of the tree.

After establishment of the inheritance relationship, the new configuration at the higher level (parent) 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.

	NOTE: The extended address pools on a DHCP server are independent of each other and no inheritance relationship exists among them. IP address lease durations are not inherited.

The DHCP server observes the following principles to select an address pool when assigning an IP address to a client:

1. 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, see "Configuring static address allocation."

2. 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, see "Configuring dynamic address allocation for an extended address pool."

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 subnet), 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 parent address pool to the client. For the configuration of such address pool, see "Configuring dynamic address allocation."

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 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.

	NOTE: Keep the IP addresses for dynamic allocation within the subnet where the interface of the DHCP server or DHCP relay agent resides to avoid wrong IP address allocation.