Installing and Configuring DHCP

If you have worked with Windows NT at all, you are probably familiar with Dynamic Host Configuration Protocol (DHCP) — a server service that enables the server to dynamically assign IP addresses to network clients. Because Windows 2000 networking has TCP/IP as its foundation, DHCP also plays an important role in Windows 2000 networks.

For the exam, you need to know how to install and configure DHCP on a Windows 2000 server, as well as how to manage its operations.

What is DHCP, anyway?

DHCP (Dynamic Host Configuration Protocol) is a server service that dynamically assigns, or leases, IP addresses and related IP information to network clients. At first glance, this may not seem like an important task. However, you have to remember that, on a TCP/IP network, each network client must have a unique IP address and an appropriate subnet mask. Without these items, a client cannot communicate on the network. For example, if two clients have the same IP address, neither will be able to communicate on the network.

Back in its early days, TCP/IP gained the reputation of being a high-overhead protocol — it required more configuration than other networking protocols. The prospect of having to visit each client machine and manually enter a correct IP address and subnet mask without making a duplication error was enough to give network administrators severe panic attacks.

DHCP handles all this work automatically. Each client gets a unique IP address, subnet mask, and other IP information such as default gateways and the IP addresses of WINS (Windows Internet Name Service) and DNS (Domain Name System) servers. DHCP makes certain that no clients have duplicate addresses, and this entire process is invisible to network administrators and network users. As you can see, DHCP is very important, and the exam expects you to know how to install and configure it.

How does DHCP work?

DHCP works by leasing IP addresses and IP information to network clients for a period of time. For the lease to happen, the following negotiation process occurs:

1. During the boot process, a client computer that is configured as a DHCP client sends out a broadcast packet called DHCPDISCOVER. This Discover packet contains the client's computer name and Media Access Control (MAC) address so the DHCP servers can respond to it. Basically, the Discover packet says, "I'm looking for a DHCP server who can lease an IP address."

2. DHCP servers on the network respond to the broadcast with a DHCPOFFER. In essence, the DHCPOFFER says, "I am a DHCP server and I have a lease for you." If several DHCP servers respond to the request, the client accepts the first offer that it receives.

3. The client responds via a broadcast message called a DHCPREQUEST. This message basically says, "I accept your lease offer and would like an IP address." If other DHCP servers made offers, they also see their lease offers were not accepted by the broadcast message, so they rescind their offers. (They must not like getting snubbed by a client computer.)

4. The DHCP server whose offer was accepted responds with a DHCPACK message, which acknowledges the lease acceptance and contains the client's IP address lease as well as other IP addressing information that you configure the server to provide. The client is now a TCP/IP client and can participate on the network.

Keep in mind that a lease is for a period of time. Typically, a client can keep its IP address for several days (or whatever you configure). When half the lease time expires, the client attempts to renew its lease for the IP address. After a client obtains the lease for an IP address, it attempts to keep the lease by renewing it over and over. If unsuccessful, the client simply must get a new IP address lease.

Reviewing important DHCP terms

You should memorize the DHCP terms listed in Table 1, because you need to know them for the exam.

Table 1: Important DHCP Terms

DHCP Term	What It Means
Scope	A full range of IP addresses that can be leased from a particular DHCP server.
Superscope	A grouping of scopes used to support logical IP subnets that exist on one physical IP subnet (called a multinet).
Multicast Scope	A scope that contains multicast IP addresses, which treat multicast clients as a group. Multicast is an extension of DHCP and uses a multicast address range of 224.0.0.0 to 239.255.255.255.
Address Pool	The IP addresses in a scope that are available for lease.
Exclusion Range	A group of IP address in the scope that are excluded from leasing. Excluded addresses are normally used to give hardware devices, such as routers, a static IP address.
Reservation	A means for assigning a permanent IP address to a particular client, server, or hardware device. Reservations are typically made for servers or hardware devices that need a static IP address.
Lease	The amount of time that a client may use an IP address before the client must re-lease the IP address or request another one.

Installing DHCP

As with other networking components in Windows 2000 Server, you can install DHCP in either of two ways:

• Using Add/Remove Programs in the Control Panel

• Using the Configure Your Server tool

Like most other Windows 2000 components, DHCP functions as a Microsoft Management Console (MMC) snap-in. After you install DHCP, you must configure the service for operation.

To open the DHCP Manager, click Start --> Programs --> Administrative Tools --> DHCP. The right pane within the snap-in tells you that you must configure the service.

DHCP does not begin leasing IP addresses and it is not functional until an administrator configures it.

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