DHCP: IP Addressing Automation

DHCP, the Dynamic Host Configuration Protocol, is used to dynamically assign IP addresses and other configuration settings to systems as they boot. This allows clients to be configured automatically at startup, thus reducing installation administration. DHCP also allows a large group of clients to share a smaller pool of IP addresses, if only a fraction of those clients needs to be connected to the Internet at any given time.

What is DHCP?

DHCP is a service that Windows Server 2003 can deliver. In other words, a Windows Server 2003 can run DHCP server software to manage IP addresses and configuration information for just about any type of TCP/IP client.

DHCP manages IP address distribution using leases. When a new system configured to use DHCP comes online and requests configuration data, an IP address is leased to that system (each lease lasts three days by default). When the duration of the lease is half- expired , the client can request a lease renewal for another three days. If that request is denied or goes unanswered, the renewal request is repeated when 87.5 percent and 100 percent of the lease duration has expired. If a lease expires and is not renewed, the client can't access the network until it obtains a new IP address lease. You can initiate manual lease renewals or releases by executing ipconfig /renew or ipconfig /release at the Windows 2003 command prompt.

Tip 

You can view the current state of IP configuration using the ipconfig command. Issuing the ipconfig /all more command at the command prompt displays all of a machine's IP configuration information, one screen at a time.

Is DHCP in your future?

We can think of two profound reasons why DHCP is a godsend to Windows 2003 administrators who need to use it:

• DHCP enables you to manage an entire collection of IP addresses in one place, on a single server, with little effort beyond the initial configuration of the address pool (the range of addresses that DHCP will be called upon to manage). In the old days (before DHCP), managing IP addresses usually required walking from machine to machine on a far too frequent basis.

• DHCP automates delivery of IP addresses and configuration information (including subnet mask and the default gateway addresses) to end- user machines. This makes it astonishingly easy to set up IP clients and to handle configuration changes when they must occur.

  To configure IP on a new client, all an end user (or you) must do in Windows Server 2003, Windows NT, or Windows 9 x is click the single option in the Internet Protocol (TCP/IP) Properties dialog box that reads, Obtain an IP Address Automatically. DHCP does the rest!

  When configuration changes occur, these changes are automatically introduced when IP leases are renewed. You can even cancel all existing leases and force clients to renew their leases whenever major renumbering or configuration changes require immediate updates to their IP configurations.

The ultimate reason for using DHCP is because it makes your job much easier. DHCP is recommended for all networks that use TCP/IP with ten or more clients. The first Windows Server 2003 in a domain has DHCP installed automatically, but you still need to enable and configure it properly before it will do you any good. So, if you think you may be interested in setting up a DHCP server, consult a technical resource, such as the Windows 2003 Resource Kit or TechNet, for all the details of installation and configuration.

Enough TCP/IP to choke a hippo

If this chapter whets your appetite for TCP/IP, you can obtain more details and information from the following great resources:

• TCP/IP with Windows NT Illustrated by Teresa Bisaillon and Brad Werne (published by McGraw-Hill Professional Publishing)

• TCP/IP Illustrated, Volumes 1,2, and 3 by W. Richard Stevens (published by Addison-Wesley)

• Internetworking with TCP/IP, Volumes I, II, and III by Douglas E. Comer (published by Prentice Hall)

• TCP/IP For Dummies , 4th Edition, by Candace Leiden, Marshall Wilensky, and John Landry (published by Wiley Publishing, Inc.)