Droms R. The DHCP handbook
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FIGURE 13.3 Starting the DHCP manager.
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FIGURE 13.4 Adding a new DHCP server.
Configuring Scopes
To configure the Microsoft server, you need to start by defining and configuring the
scopes the server will manage through the DHCP manager. Start the DHCP manager
and select the entry for the DHCP server you want to configure. This section uses the
dhcp-example entry, which identifies the DHCP server on the computer used in this
chapter, as an example. Select
New Scope from the Action menu. The DHCP manager
begins the New Scope Wizard; through this wizard you are prompted to enter the
name and an optional comment for the scope, the available addresses in the scope,
and the subnet mask. Figure 13.5 shows the dialog box you use to enter the name
and the description of a new scope.
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FIGURE 13.5 Creating a new scope in the DHCP manager.
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Naming Scopes
Each scope defined for the Microsoft DHCP server can have an associated name. You
use this name to identify the scope in the list of available scopes displayed by the
DHCP manager. Because the DHCP manager displays the scopes in alphabetical
order, you might want to define a naming convention that groups related scopes
together by assigning names with the same prefix to those scopes.
Available and Excluded Addresses in a Scope
Within a scope, you define the set of addresses that are available for assignment by
the DHCP server. You can also designate a list of excluded addresses that the DHCP
server should not assign. You can define the list of available addresses so that it
contains just the addresses in the scope that are available for the server to assign.
However, each scope definition can contain only one list of available addresses; each
scope definition must be contiguous and specified by the first and last addresses in
the list. Therefore, if you want the list of available addresses to include noncontigu-
ous addresses, you must declare the list of available addresses to span all the avail-
able addresses and then specifically exclude some addresses.
To illustrate how to define available addresses to the Microsoft DHCP server, suppose
a network segment is defined to have IP address 192.168.11.0 and subnet mask
255.255.255.0 and that the DHCP server is to assign addresses
192.168.11.10–192.168.11.19 and 192.168.11.30–192.168.11.39. In this example the
corresponding declaration in the configuration file for the ISC server is shown in
Example 13.1.
Example 13.1
subnet 192.168.11.0 subnet-mask 255.255.255.0 {
range 192.168.11.10-192.168.11.19;
range 192.168.11.30-192.168.11.39;
}
Figure 13.6 illustrates the definition of the list of available addresses through the
New Scope Wizard. Note that the subnet mask for the scope is also defined in this
dialog box.
You can configure the Microsoft DHCP server for the available addresses shown in
Figure 13.6 by setting the range to 192.168.11.10–192.168.11.39 and then putting
the range 192.168.11.20–192.168.11.29 in the list of excluded addresses. Figure 13.7
shows the next dialog box in the New Scope Wizard, in which the addresses
192.168.11.20 through 192.168.11.29 have been excluded from assignment.
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FIGURE 13.6 Defining a scope and available addresses with the DHCP manager.
Configuring DHCP Servers 209
FIGURE 13.7 Excluding addresses from assignment.
A more general configuration of the Microsoft DHCP server in this case is to declare
the entire range of addresses in the subnet, 192.168.11.1–192.168.11.254, as avail-
able addresses, and then to explicitly add all the addresses that are not available for
DHCP to the list of excluded addresses. With this configuration for the scope, you
can change the list of available addresses by modifying the list of excluded addresses,
without changing the range of available addresses in the scope. The list of excluded
addresses in this example is as follows:
192.168.11.1–192.168.11.9
192.168.11.20–192.168.11.29
192.168.11.40–192.168.11.254
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Setting Lease Duration
The next dialog box in the New Scope Wizard asks for the duration of leases associ-
ated with addresses assigned from the scope (see Figure 13.8). The default value is
eight days. The appropriate duration for leases depends on your specific network
requirements and design, so you will probably want to review this default duration
to see if it is appropriate. Chapter 19, “Tuning a DHCP Service,” gives detailed advice
on choosing lease durations. In Figure 13.8, the lease duration is set to 90 days, illus-
trating the configuration for the
genericstartup.com
network discussed in Chapter 3,
“Configuring the DHCP Server.”
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FIGURE 13.8 Selecting lease duration.
Configuring Other Options
The New Scope Wizard allows you to configure other configuration parameters, or
options, to be returned to hosts that are assigned addresses in the scope. The first of
these options is the list of default gateways (or routers) for the scope, as shown in
Figure 13.9.
The next configuration parameters to set are the domain name and a list of DNS
servers for hosts in the scope to use. The dialog box you use to set these parameters
is illustrated in Figure 13.10.
Finally, the New Scope Wizard prompts you to enter any WINS servers for the scope,
as shown in Figure 13.11.
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FIGURE 13.11 Identifying WINS servers for the scope.
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FIGURE 13.9 Selecting default gateways.
FIGURE 13.10 Setting the domain name and the list of DNS servers.
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The final New Scope Wizard dialog box asks if you want to activate the scope (see
Figure 13.12). As soon as you activate the scope, the DHCP server begins assigning
addresses from that scope, so be sure the scope is completely configured and there
are no other DHCP servers assigning the addresses in the scope before you activate it.
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FIGURE 13.12 Activating a new scope.
Configuring Superscopes
The Microsoft Windows 2000 DHCP server includes the ability to define superscopes,
through which you can associate multiple scopes with a single network segment.
Superscopes are equivalent to the
shared-network declaration in the ISC server
configuration file. You use a superscope when you’ve assigned multiple IP subnets to
a single network segment.
The primary function of superscopes is address assignment. The DHCP server groups
together all the available addresses from the scopes in a superscope into a single pool
of available addresses for the network segment. It finds an unassigned address from
one of the scopes in the superscope to assign to a client that needs an address from
the superscope.
Client options are managed through the separate scopes in a superscope. The DHCP
manager does not include a mechanism for defining options that are common to all
scopes in a superscope, nor does it give you a way to examine the list of available
addresses across the superscope.
NOTE
The DHCP administrator must make sure that options are defined consistently across the
scopes within a superscope. Options such as the DNS server or NTP server options should
usually be defined uniformly across the superscope. Some options, such as the default
routers option, should be configured differently for each scope.
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You manage superscopes through the DHCP manager. To create a superscope, select a
DHCP server and then select Action, the New Superscope. The DHCP manager starts
the New Superscope Wizard for creating superscopes, as shown in Figure 13.13.
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FIGURE 13.13 The New Superscope Wizard in the DHCP manager.
In the first dialog box of the New Superscope Wizard, you should enter the name of
the new superscope. The next dialog box, shown in Figure 13.14, displays a list of
scopes that you can add to the new superscope. Add the desired scopes to the super-
scope and click Next.
FIGURE 13.14 Adding scopes to a superscope.
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Adding a Scope to an Existing Superscope
To add a scope to an existing superscope, first select the scope in the DHCP manager,
and then choose Add to Superscope from the Action menu. Then, select the super-
scope to which the scope should be added and click OK. To remove a scope from a
superscope, first select the superscope and then the scope to be removed. Choose
Remove from Superscope from the Action menu and click Yes.
Configuring Reservations
You can permanently assign a specific address to a client by establishing a reservation,
which is equivalent to a host declaration in the configuration file for the ISC DHCP
server. A reservation is appropriate for a client that should always use the same IP
address, such as a computer that provides a service such as DNS. A reservation is
established as part of the scope to which the reserved address belongs. The address
assigned through a reservation must be in the range of available addresses for the
scope.
You use the DHCP manager to create, change, and delete reservations. To create a
reservation, select the scope to which the reservation is to be added, and then
select Reservations under that scope. Select Action, New Reservation, and the
Server manager displays a dialog box that requests the following information (see
Figure 13.15):
Information Requested Description
Reservation name A name used to identify the reservation by the DHCP
manager.
IP address The address to be reserved; it must be an unassigned
address within the range of addresses for a scope.
MAC address The Media Access Control (MAC) address (usually the
Ethernet hardware address) of the client for which this
reservation is defined.
Description Optional text to describe the client and reservation.
You can also select whether this reservation should apply to DHCP requests, BOOTP
requests, or both. After filling in the values for the requested fields, click the Add
button to add the reservation to the DHCP configuration database.
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FIGURE 13.15 Defining a reservation for a DHCP client.
Configuring Options
As with the ISC server, you can configure the Microsoft server with rules that govern
the values for options that the server passes to DHCP clients. However, you can
configure the Microsoft server with only three types of rules:
•Values for options that are defined to apply to specific clients that are preas-
signed specific addresses
•Values for options that are defined to apply to clients in a specific scope
•Values that are to be applied to every client the server manages
To select a specific option for configuration, the DHCP manager displays a pull-
down menu with a list of all available DHCP options. In this list, the options are
identified by both option number and name. Table D.1 in Appendix D, “DHCP
Options Summary,” lists all the DHCP options and the Microsoft DHCP server’s
name for each option.
Scope Options
Scope options specify values for options to be returned to any client within the scope
for which the option is defined. That is, a client receives the option values from the
scope to which the client belongs.
To configure a scope option, select a scope in the DHCP manager and click Scope
Options. Select Action, Configure Options. The DHCP manager displays a window in
which you can choose an option and the value to be returned to clients in that
scope. After you’ve selected an option for a scope, it appears in the DHCP manager’s
main window when the scope is selected. Figure 13.16 shows the configuration of
the Routers option for a single scope.
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