Through DHCP, a specific IP address can be reserved for a computer or other IP addressable device on a network. Reserving selected IP addresses for special-function devices on a network ensures that DHCP does not duplicate or reassign the address. The following types of devices and computers can use reservations:
Other servers running Windows on a network that require static IP addresses, such as WINS servers.
Any print servers that use TCP/IP print services.
UNIX or other clients that use IP addresses assigned by another TCP/IP configuration method.
Any DNS servers.
Each reservation requires a unique identifier for the address of the reserved device, which corresponds to the media access control (MAC) or physical address for the DHCP client. Ethernet addresses, made up of a unique sequence of hexadecimal numbers, identify the network adapter hardware for each network-connected device.
Note: To obtain MAC addresses on Windows NT–based clients, type ipconfig /all at the command prompt and view the Physical Address field. For Windows 9x–based clients, run Winipcfg.exe, and view the Adapter Address field.
Using BOOTP Tables
The DHCP Server service offers BOOTP support in the form of pointer records contained in the BOOTP table. BOOTP, which preceded DHCP, enables diskless clients to obtain their own IP addresses and other boot information needed for network startup. Many Windows NT–based installations do not require BOOTP, so the BOOTP table does not need to be configured.
BOOTP allows diskless clients to use User Datagram Protocol (UDP) packets to request and retrieve an IP address and a small boot image file from a Trivial File Transfer Protocol (TFTP) server.
Data stored in the BOOTP table is returned to any BOOTP network clients that broadcast a BOOTP request message. If a BOOTP record exists in the BOOTP table, the DHCP server returns a BOOTP message to the requesting BOOTP client. If no BOOTP records are configured, the DHCP Server service silently drops BOOTP request messages.
The reply message returned by the DHCP Server service indicates the name and location of a TFTP server on the network, which the client can contact to retrieve its boot image file. Each record in the BOOTP table contains the following three fields, which in turn contain the information returned to the BOOTP client:
The Boot Image field identifies the generic file name of the boot file requested based on the BOOTP client’s computer type.
The File Name field identifies the full path of the boot file returned by TFTP by the BOOTP server to the client.
The File Server field identifies the TFTP server used to source the boot file.
You can use the DHCP snap-in to add, remove, and edit records in the BOOTP table.
Although both BOOTP and DHCP allocate IP addresses to clients during startup, they use different methods of allocation. BOOTP typically provides fixed allocation of a single IP address for each client, permanently reserving this address in the BOOTP server database. DHCP typically provides dynamic, leased allocation of available IP addresses, reserving each DHCP client address temporarily in the DHCP server database.
Best practices optimize the functionality and performance of the DHCP service in Windows Server 2003. These are described below.
Set Appropriate Lease Durations
Because lease renewal processes can affect the performance of DHCP clients and the network, selecting a different lease duration can improve overall network performance. The following guidelines will help administrators determine the best configuration for their network.
Lengthening Lease Duration for Large, Fixed Networks
It’s a good practice to increase scope lease length for large, stable, fixed networks that have plentiful scope address space. Increasing the lease duration lowers the frequency of lease renewal queries between clients and the DHCP server, thus reducing associated network traffic. Most useful for larger routed networks, lengthening the default lease period from seven to perhaps 21 days reduces DHCP-related network broadcast traffic, particularly if client computers generally remain in fixed locations and scope addresses remain plentiful, such as with less than 80 percent in use.
Shortening Lease Duration for Variable Networks with Fewer IP Addresses
By contrast, for networks that have fewer IP addresses available and either client configurations or network locations that change, it’s best to reduce the lease duration, which increases the rate at which addresses return to the available address pool for reassignment to new clients by the DHCP server. A sales organization, for example, which might issue laptop computers to traveling employees, might find this practice especially beneficial. When the Routing and Remote Access service supports dial-up clients on the network, adjust lease time on scopes that serve these clients to less than the default of eight days.
Most network configurations require a mixture of lease durations. With a single segment where laptops come and go, shortening the lease on that scope might improve performance, while other parts of a network with a stable body of clients could set the lease duration somewhat higher.
It’s a best practice to use a client reservation to ensure that a DHCP client computer always receives the same IP address lease at startup. If you have more than one DHCP server that is can be reached by a reserved client, add the reservation at each of your other DHCP servers. This practice allows the other DHCP servers to honor the client IP address reservation made for the reserved client. Although the DHCP server will act upon the client reservation when the reserved address is part of the available address pool, having the same reservation for the same client on multiple servers will not usually cause any problems.