Allocation of IP addresses, either temporary or permanent, is one of the two primary services provided by DHCP. Basically, the client requests an IP address, and the DHCP server (or collection of DHCP servers) provides an address and guarantees not to give that address to another client within a specified time. Additionally, the server tries to return the same address to the client each time the client requests an address. The period of time over which an IP address is allocated to a client is called a lease .
DHCP supports three methods of IP address allocation:
A network can use one or more of these methods. The network administrator decides which methods to use.
Dynamic BOOTP enables a DHCP server to assign permanent addresses to BOOTP clients from a pool of addresses. No manual configuration of the client is required prior to address allocation.
Dynamic DHCP allocation is the only method enabling automatic reuse of addresses no longer required by a client. Dynamic DHCP allocation is useful for assigning an address to a client that will be connected temporarily to the network or for sharing a limited number of IP addresses among a group of clients that do not require permanently assigned IP addresses.
Dynamic DHCP allocation is also useful for assigning an IP address to a new client installed on a network on which IP addresses are scarce and must be reclaimed when older hosts are removed. An additional benefit to dynamic DHCP allocation is that when a client's lease is renewed, the DHCP server refreshes the client's configuration.
Manual or static allocation enables a network administrator to use the DNS/DHCP Management Console to assign addresses to DHCP or BOOTP clients. A specific IP address is assigned to the client based on an identifier such as the client's hardware or MAC address.
Manual allocation of DHCP eliminates the error-prone method of manually configuring hosts with IP addresses in networks for which IP address management outside a DHCP mechanism is desired. Manual allocation can be permanent or set to expire at a future time. When you perform manual allocation, you can also create corresponding DNS Resource Records, thereby eliminating another error-prone activity.
A client acquires a lease for a fixed period of time. The length of the lease could be a number of hours or days, or it could be for an indefinite period.
After having been granted a lease for an IP address, a client can issue a request to extend its lease. The client can also issue a message to the server to release the address back to the server when the address is no longer required.
If a network has a scarcity of IP addresses and must reassign them, the DHCP server will reassign an address when the lease has expired. The server uses configuration information to choose addresses to reuse. For example, the server might choose the least recently assigned address for reassignment. After receiving an address assignment, the host determines whether the address is in use by another host before accepting the address.
IMPORTANT: Address duplication sometimes occurs with Windows 95 clients If a Windows 95 client receives a response indicating that the assigned address is in use by another device, a message is displayed indicating the IP address conflict. However, the client does not send a DHCPDECLINE message as required by RFC 1534, section 4.4.1.
To minimize the chance of address duplication, the DHCP server can be configured to ping an address to test its validity prior to assigning it to a host. If the server receives a response from another device (indicating ownership of the address), the current address assignment is withdrawn so that another address can be assigned to the host.