Besides receiving an IP address, subnet mask, and default gateway from a DHCP server, DHCP clients can receive DHCP options that supply many different configuration parameters. Deciding which DHCP options to include can be determined by asking the following questions:
Which DHCP options do DHCP clients in this scope require?
What DHCP clients are in use on the network?
Which DHCP options do the DHCP clients support?
Determining which options are required is relatively simple, unless there are applications in use that have special needs. Besides determining which options to use, an administrator must determine the values of those options as well. For example, an administrator wants the DHCP clients to receive DNS server addresses. For load balancing, each subnet has a different secondary DNS server to service client requests. In this case, the administrator must supply the correct IP address for each subnet’s DNS server.
Next, an administrator must determine which DHCP clients are in use on the network. Since Microsoft operating systems are the most prevalent on most corporate desktops and laptops, it can pretty much be said that almost every network includes some Microsoft DHCP clients.
But there may be other types of DHCP clients as well, such as Unix, Linux, or Macintosh. Although these operating systems can all be DHCP clients, their implementations of DHCP vary. For example, they may not support certain DHCP options, such as WINS servers. The DHCP server in Windows 2000 supports all DHCP options defined in RFC2132. If you have a non-Microsoft DHCP client, you can configure the Windows 2000 DHCP Server to supply any DHCP option that the client can support. Refer to your non-Microsoft DHCP client’s documentation for a complete list of supported DHCP options.
There may also be network devices that support DHCP, such as network printers. Deciding whether or not to use DHCP for network printers is a matter of choice; most administrators prefer to assign static addresses to the printers. This way IP addresses for the printers are always known, thus simplifying management and troubleshooting. However, DHCP can be used with network printers by using the manual allocation addressing method. By creating an address reservation using the printer’s MAC address, the printer can receive other configuration information that may change from time to time, such as name server addresses.
Determining the DHCP options that the DHCP clients support can be a bit trickier. I will briefly describe which options are supported by most of the major operating systems. An administrator should always refer to the operating system’s documentation to ascertain which options are supported.
Microsoft-based clients request the following DHCP options, described in Chapter 3, and defined as properties of the scope:
Renewal Time Option (58)
Rebinding Time Option (59)
IP Address Lease Time Option (51)
Server Identifier Option (54)
Subnet Mask Option (1)
Microsoft-based clients will also request the following DHCP options:
Routers Option (3)
Domain Name Option (15)
Domain Name Servers Option (6)
NetBIOS Name Servers Option (44)
NetBIOS Node Type Option (46)
NetBIOS Scope Option (47)
Appendix A lists all currently available DHCP options. Third-party clients such as Unix, Linux, and MacOS may also support these DHCP options.