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DHCP and TCP/IP  «Prev  Next»
Lesson 3 DHCP features
ObjectiveIdentify DHCP configuration features supported by the DHCP service.

DHCP features

Identify DHCP configuration features supported by DHCP Service

The Windows 2000 DCHP Server has much in common with the DHCP Server you may have used on your Windows NT 4.0 networks. If you are an experienced UNIX administrator, you will find that the Windows 2000 DHCP Server is much more robust than what you might have expected. The Windows 2000 DHCP Server is RFC-compliant and integrates with other important network services in a Windows 2000 environment.
The Windows 2000 DHCP Server also has several new features, including integration with the Active Directory, support for vendor and user options classes, and support for MADCAP (Multicast Dynamic Client Allocation Protocol).
These features include:
  1. RFC compliance
  2. DNS integration
  3. Active Directory™ integration
  4. Microsoft vendor-specific options
  5. Microsoft support for multicast IP address allocation
  6. We will explain each of these features in the table below.

RFC compliance

There are three primary management features that DHCP supports and which are RFC compliant. This table clarifies what these features are and how they work:
Feature Definition An example of how it works
Scope A Scope is a group of IP addresses from a single subnet that the DHCP Server uses to assign IP addresses to members of the subnet. If a computer is on network ID, a Scope would be created that had IP addresses The DHCP Client would then be assigned and an IP address from the group of IP addresses included in the Scope.
Superscope A Superscope is a combination of multiple Scopes. Superscopes allow you to increase the number of network clients that are on the same physical segment. You cannot do this with a single Class C Scope.
DHCP options DHCP options represent "optional" parameters that the DHCP Server can assign to network clients. There are four levels of DHCP options: Server options, Scope options, Class options (User or Vendor Class), and Reserved Client options. The DHCP Server must assign at least the IP address and subnet mask to all DHCP Clients. However, you have the option of assigning other information to the DHCP Clients, such as the IP address of the DHCP Client's default gateway or WINS Server. These assignments are referred to as DHCP options.

Note: These management features comply with RFCs 951, 2131, and 2132. You can find these RFCs listed with these features on the Resources page.

DNS integration

DHCP and DNS integration allows earlier versions of Windows-based clients and non-Microsoft DHCP Clients to have their records automatically updated in the DNS database by the DHCP Server.
The Windows 2000 DHCP Server can update DNS record information for Windows 2000 DHCP Clients. By default, the Windows 2000 DHCP Client updates its own Host (A) address record and the DHCP Server updates the DHCP Client's Pointer (PTR) record. The DHCP Server can also act as a "Proxy" for down-level DHCP Clients that cannot register their IP addressing information directly with the Windows 2000 Dynamic DNS Server. The DHCP will dynamically register the down-level clients Host (A) record and Pointer (PTR) record information for the down-level client.

Active Directory integration

The integration of the DHCP service with Active Directory allows DHCP Servers to be authorized within Active Directory. Windows 2000-based DHCP Servers do not start unless authorized. Active Directory integration prevents the introduction of "rogue" DHCP Servers onto the network. Because DHCP Servers must be authorized in the Active Directory before they can start their DHCP Server service, machines that have not been authorized by an administrator will not be able to function as DHCP Servers. The major drawback of the DHCP authorization scheme is that it works only with Windows 2000 DHCP Servers. If someone were to introduce a Windows NT 4.0 DHCP Server onto the network, it would not be shut down because it is not Active-Directory aware.

Microsoft's vendor-specific options

In addition to RFC-compliant DHCP options, Microsoft supports several RFC-defined, vendor-specific options.
This Slide Show describes these vendor options:
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  1. Disable NetBios over TCP/IP (NetBT). This option is used to enable or disable NetBT on Windows 2000 DHCP Clients. Earlier Windows clients require NetBT and do not support this option.
  2. The experienced administrator will recognize this as a tremendous benefit. NetBIOS traffic represents much of the broadcast overhead on Microsoft networks. By disabling NetBIOS over TCP/IP, you can eliminate NetBIOS broadcast traffic on your network.
  3. Another plus is that after NetBT is disabled, the browser service no longer functions. This eliminates complaints that users can't see something on the network. Users can access all network resources via the Active Directory instead of the My Network Places applet.
  4. This option is used to control whether DHCP Server-enabled computers send a release for their current DHCP lease to the DHCP Server when the computer shuts down.
  5. This Microsoft vendor-specific option enables DHCP Clients to release their IP Addresses on shutdown, which is very useful if you decide to change DHCP options frequently or are short of IP addresses.
  6. If this option is set, the DHCP Client uses the value configured here as the base metric for its default gateways. The computer will use the default gateway with the lowest hop count By configuring the base metric, you can control which gateways the DHCP Client will use first and in what order.

Note that some DHCP Server options supported by Windows 2000 are only recognized by a Windows 2000 DHCP Client. Previous versions of Windows clients and non-Microsoft clients might not support all options. This is true of the Vendor and User Class options, because the DHCP Client must be able to send information about what class it belongs to in order for the DHCP Server to respond with the appropriate Class options.
These vendor-specific options are defined in RFC 2132. You can find this RFC on the Resources page.

Microsoft® support for multicast IP Address Allocation

As already mentioned, the DHCP service in Windows 2000 also supports MADCAP, in addition to DHCP. These protocols are supported independently by the DHCP service. MADCAP is used to enable multicast clients to join multicast groups. To take advantage of this multicast support, network applications, which utilize multicast addresses, must be installed on client machines and take advantage of the appropriate APIs. The DHCP service in Windows 2000 supports multicast scopes independently of the DHCP scopes. The independence of these scopes is important because DHCP Clients typically obtain a single IP address. However, multicast clients can obtain a multicast address in addition to their "regular" IP address, which they use for all other network communications.

Note: For more information on MADCAP and support for multicast groups, see the reference to this subject on the Resources page.
The next lesson deals with integration benefits. More specifically, you will learn how the DHCP service integrates with other services in Windows 2000. MADCAP: A protocol that automates distribution of multicast address configurations for network clients.