New Features

The Windows 2000 DHCP service provides the following new features:

• Enhanced performance monitoring and server reporting capabilities

  New System Monitor counters have been added to Windows 2000 Server to specifically monitor DHCP server performance on your network. Additionally, DHCP Manager now provides enhanced server reporting through graphical display of current states for servers, scopes, and clients. For example, icons visually represent whether a server is disconnected, or if it has leased over 90 percent of its available addresses.

• Expanded support for multicast scopes and superscopes

  Multicast scopes now allow multicast-aware applications to lease Class D–type IP addresses (224.0.0.0 to 239.255.255.255) for participation in multicast groups.

• Support for user-specific and vendor-specific DHCP options

  This allows the separation and distribution of options for clients with similar or special configuration needs. For example, you might assign all DHCP-enabled clients on the same floor of your building to the same option class. You could use this class (configured with the same DHCP Class ID value) to distribute other option data during the lease process, overriding any scope or global default options.

• Integration of DHCP with DNS

  A DHCP server can enable dynamic updates in the DNS namespace for any DHCP clients that support these updates. Scope clients can then use DNS with dynamic updates to update their computer name–to–IP address mapping information whenever changes occur to their DHCP-assigned address.

• Rogue DHCP server detection

  This prevents rogue (unauthorized) DHCP servers from joining an existing DHCP network in which Windows 2000 Server and Active Directory are deployed. A DHCP server object is created in Active Directory, which lists the IP addresses of servers that are authorized to provide DHCP services to the network. When a DHCP server attempts to start on the network, Active Directory is queried and the server computer's IP address is compared to the list of authorized DHCP servers. If a match is found, the server computer is authorized as a DHCP server and is allowed to complete the system startup. If a match is not found, the server is identified as rogue, and the DHCP service is automatically shut down.

• Dynamic support for BOOTP clients

  Dynamic BOOTP is an extension of the BOOTP protocol, which permits the DHCP server to configure BOOTP clients without having to use explicit, fixed-address configuration. This feature reduces administration of large BOOTP networks by allowing automatic distribution of IP address much the same way that DHCP does.

• Read-only console access to DHCP Manager

  This feature provides a special-purpose local group, the DHCP Users group, which is automatically added when the DHCP service is installed. By adding members to this group, you can provide read-only access to information related to the DHCP service on the server computer. Using DHCP Manager, users in this group can view, but not modify, information and properties stored on the specified DHCP server.

DHCP Client Support

The term client is used to describe a networked computer that requests and uses the DHCP services offered by a DHCP server. Any Windows-based computer, or other network-enabled device that supports the ability to communicate with a DHCP server (in compliance with RFC 2132), can be configured as a DHCP client.

DHCP client support is provided for computers running under any of the following Microsoft operating systems:

• Microsoft® Windows NT® Workstation (all released versions)
• Microsoft® Windows NT® Server (all released versions)
• Microsoft® Windows® 98
• Microsoft® Windows® 95
• Microsoft® Windows® for Workgroups version 3.11 (with the Microsoft 32-bit TCP/IP VxD installed)
• Microsoft® Network Client version 3.0 for MS-DOS (with the real-mode TCP/IP driver installed)
• LAN Manager version 2.2c

IP Auto-Configuration

Windows 2000–based clients can automatically configure an IP address and subnet mask if a DHCP server is unavailable at system start time. This feature, Automatic Private IP Addressing (APIPA), is useful for clients on small private networks, such as a small-business office, a home office, or a remote access client.

The Windows 2000 DHCP client service goes through the following process to auto-configure the client:

1. The DHCP client attempts to locate a DHCP server and obtain an address and configuration.
2. If a DHCP server cannot be found or does not respond, the DHCP client auto-configures its IP address and subnet mask using a selected address from the Microsoft-reserved Class B network, 169.254.0.0, with the subnet mask 255.255.0.0. The DHCP client tests for an address conflict to make sure that the IP address it has chosen is not already in use on the network. If a conflict is found, the client selects another IP address. The client will retry auto-configuration for up to 10 addresses.
3. Once the DHCP client succeeds in self-selecting an address, it configures its network interface with the IP address. The client then continues, in the background, to check for a DHCP server every 5 minutes. If a DHCP server is found later, the client abandons its auto-configured information. The DHCP client then uses an address offered by the DHCP server (and any other provided DHCP option information) to update its IP configuration settings.

If the DHCP client had previously obtained a lease from a DHCP server:

1. If the client's lease is still valid (not expired) at system start time, the client will try to renew its lease.
2. If, during the renewal attempt, the client fails to locate any DHCP server, it will attempt to ping the default gateway listed in the lease, and proceed in one of the following ways:
   • If the ping is successful, the DHCP client assumes that it is still located on the same network where it obtained its current lease, and continue to use the lease. By default, the client will then attempt, in the background, to renew its lease when 50 percent of its assigned lease time has expired.
   • If the ping fails, the DHCP client assumes that it has been moved to a network where DHCP services are not available. The client then auto-configures its IP address as described previously. Once the client is auto-configured, every 5 minutes it attempts to locate a DHCP server and obtain a lease.

Local Storage

Microsoft DHCP supports local storage, allowing clients to store DHCP information on their own hard disks. Local storage is useful because when the client system starts, it first attempts to renew the lease of the same IP address. Local storage also means that a client can be shut down and restarted using its previously leased address and configuration, even if the DHCP server is unreachable or offline at the time the client computer is restarted. Local storage also enables the ability to perform IP auto-configuration.

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