Design Option 2: DHCP/BOOTP Reservations
Updated: November 10, 2008
Applies To: Windows Server 2008
With client reservations, it is possible to reserve a specific IP address for permanent use by a DHCP client. To create a reservation, the hardware address of the client network adapter (also known as the MAC address) is associated with a particular IP address in the DHCP scope. The reserved client is assigned the IP address associated with its MAC address each time it renews its lease with the DHCP server.
Reserved clients can have DHCP options configured specifically for their use. When options are configured for a reserved client, these values override any option type parameters distributed through server-based, scope-based, or class-based options assignment.
If multiple DHCP servers are configured with a scope that covers a range of the reserved IP addresses, the client reservation must be made and duplicated at each of the DHCP servers; otherwise, the reserved client computer can receive a different IP address based on the responding DHCP server. You can create reservations using any IP address in the scope's address range, even if the IP address is within an exclusion range. Using this design, when the 80/20 rule is implemented and all addresses in the scope are excluded (80 percent at one server, 20 percent at the other), reservations still function properly. For more information about the 80/20 rule and other DHCP best practices, see the DHCP Server Operations Guide.
The advantages of DHCP/BOOTP reservations include:
Centralized administration: Reservations can be centrally planned and administered; they do not require support staff to visit each device that needs a fixed address.
Fixed address and dynamic information: Reservations allow configuration information, such as default gateway and DNS server addresses, to be changed through the DHCP scope properties, even though the IP address itself remains static through the reservation.
The disadvantages of DHCP/BOOTP reservations include:
Administrative overhead: Some work is needed at the client to determine the MAC address to use for reservation. If the network adapter is replaced, the reservation must be reconfigured because the MAC address will have changed (unless the device has a configurable MAC address).
Reservations must be defined globally: Address reservations must be configured on all DHCP servers across the network. Otherwise, if the main DHCP server is unavailable, a client might be assigned the reserved address from a backup server on another subnet.
Not suitable for critical services: Reserved addresses can only be leased if DHCP is available; therefore, it might be better to use a manually configured fixed address for critical devices.
Not suitable for servers running Windows 2000, Windows Server 2003, and Windows Server 2008: On these operating systems, network services, such as DNS and Active Directory domain controllers, cannot be properly configured unless their IP addresses are manually assigned.