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Hour 16. Using the Dynamic Host Configuration Protocol

What You’ll Learn in This Hour:

IP networks require that each device on the network (computer, printer, router, and so on) be assigned a unique IP address. Configuring static addresses on a large network can be both time-consuming and, in the case of poor or incomplete documentation, difficult to manage and troubleshoot. Tracking and assigning unique IP addresses became the focus of the Internet Engineering Task Force and leading software developers; the result was the development of the Dynamic Host Configuration Protocol (DHCP). This hour looks at how to deploy DHCP on a server running Windows Server 2008.

Understanding DHCP

The Dynamic Host Configuration Protocol enables you to dynamically assign IP addresses to your network computers and other devices. IP addresses are taken from a pool of addresses and are assigned to computers either permanently or for a fixed lease time.

When you consider that you must configure every client computer on an IP network with such things as an IP address, a subnet mask, a default gateway address, and a DNS server address, you can see that there is an incredible margin for error if this is done manually.

DHCP provides a dynamic environment for assigning IP addresses to computers and devices on the network. It actually simplifies much of the drudgery that would be involved in manually assigning IP addresses.

By the Way

DHCP evolved from a protocol called BOOTP, short for the Bootstrap Protocol, which was used to assign IP addresses to diskless workstations. BOOTP did not assign IP addresses dynamically, however; it pulled IP addresses from a static BOOTP file that was created by the network administrator.

A DHCP server (any Windows Server 2008 configured with the DHCP service) can supply an IP address, subnet mask, default gateway, DNS server address, and WINS server address to a DHCP client. A DHCP client is any computer or device on the network that is configured to acquire its IP address (and other TCP/IP settings) dynamically.

When a DHCP client boots up for the first time, it goes looking for an IP address. The client initializes TCP/IP (a stripped-down version) and broadcasts a DHCPDISCOVER message, which is a request for an IP lease that is sent to all DHCP servers (addressed to 255.255.255.255, meaning all nodes on the network). This broadcast message contains the client’s hostname (which, in most cases, is also the client’s NetBIOS name) and the client’s MAC hardware address.

By the Way

The MAC address is the unique hardware address burned into a ROM chip on a device such as a network interface card or a router interface by the device manufacturer.

In the next step, a DCHP server (or servers, if more than one is available) on the subnet responds with a DHCPOFFER message that includes an offered IP address, an accompanying subnet mask, and the length of the lease. The message also contains the IP address of the DHCP server, identifying the server. The DHCPOFFER message is also in the form of a broadcast because, at this point, the client does not have an IP address.

When the client receives the first DHCPOFFER message (it might receive multiple offers, but it goes with the first appropriate offer it receives), it then broadcasts a DHCPREQUEST message to all DHCP servers on the network, showing that it is accepting an offer. This broadcast message contains the IP address of the DHCP server whose offer the client accepted. Knowing which DHCP server was selected enables the other DHCP servers on the network to retract their offers and save their IP addresses for the next requesting client. (Yes, it does sound a little bit like a used car lot.)

Finally, the DHCP server that supplied the accepted offer broadcasts an acknowledgement message to the client: a DHCPACK message. This message contains a valid IP address lease and other TCP/IP configuration information. The client stores this information in its Windows Registry.

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