Creating reservations

A reservation reserves a specific IP address for a specific Media Access Control (MAC) address. The MAC address is a unique hardware-based address that identifies a network adapter (Network Interface Card, or NIC) on the network. Reservations enable a specific adapter to receive the same IP address assignment from the DHCP server, and prevent the address from being leased to any other adapter. In effect, reservations let you enjoy the flexibility offered by DHCP while still enabling you to assign a static IP address. Through reservations, you ensure that the NIC always has the same IP address, but other configuration changes can be applied dynamically (such as domain name, router, DNS servers, and so on).

Note Reservations do not assign the same IP address to a computer per se, because the reserva tion is associated with the NIC's MAC address, not the computer name. This is only a real distinction in multi-homed systems (those containing multiple NICs).

Before creating a reservation for a NIC, you need to know the NIC's MAC address. On Windows NT, Windows 2000, Windows XP, and Windows Server 2003 systems, you can use the ipconfi g command at a console prompt to view MAC addresses for NICs in the computer. Open a console prompt on the system and issue the command ipconfig / all. The command lists network configuration data for each NIC, including the MAC address.

For Windows 9x and Me systems, use the WINIPCFG utility to determine the MAC address. WINIPCFG includes the adapter address in the information it displays, along with the IP address, the gateway, and other configuration information.

When you have the MAC address of the client's NIC, open the DHCP console and then open the scope where you want to create the reservation. Right-click the Reservations node and choose New Reservation to open the New Reservation dialog box (see Figure 16-5). Use the following list as a guide to configure the reservation:

♦ Reservation name. This name appears in the DHCP console next to the reservation IP address (left pane). You can specify the computer's name, the username, or other information to help you identify the NIC for which the address is reserved.

♦ IP address. Specify the IP address within the scope to reserve for the specified NIC.

♦ MAC address. Enter the MAC address of the NIC for which the address is reserved.

♦ Description. This optional description appears in the contents pane of the DHCP console.

♦ Supported types. You can designate the type of client (DHCP, BOOTP, or both) that can use the reservation.

Figure 16-5: Reservations assign an IP address to a specific network adapter.

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