DHCP for Windows 2000

Chapter 4. Designing a DHCP Infrastructure

DHCP can quickly become an essential piece of an organization’s data network. Once set up, DHCP is usually hardly noticed, silently and faithfully performing its duties day in and day out. Unfortunately, the hardest thing with DHCP is getting it to that point.

This chapter discusses the reasons an organization would want to use DHCP, along with the many different issues that need to be considered when designing a DHCP infrastructure.

Some of these considerations include planning for IP address use. An organization needs to determine how their existing environment is used as well as what types of users and workstations exist, such as mobile users and network devices.

The needs of a DHCP client must be considered, including which DHCP options are supported by the client’s operating system and which options and their values need to be assigned.

In large-scale DHCP implementations, the topology of the network becomes a very important factor. The network topology dictates where DHCP servers and/or relay agents must be placed.

A final consideration is planning for fault tolerance. Once DHCP is implemented, it quickly becomes a service that the entire network is dependent on. Steps can be taken to ensure that DHCP will be available at all times.

Who Needs DHCP?

The Internet Engineering Task Force (IETF) created this new protocol, DHCP, to dynamically distribute IP addresses and configurations to clients. But what types of organizations would benefit from using DHCP?

Some administrators believe that having to administer yet another network service and the additional traffic it creates is an unnecessary burden. Administrators with this philosophy believe that it is easier to set up workstations and servers with static configurations that do not need to be maintained or administered.

In reality, however, any organization that wants to save the time and aggravation of manually maintaining the allocation of IP addresses would want to use DHCP. DHCP allows an administrator to standardize the IP address configurations for the entire network while dynamically maintaining the address table in a database.

Small companies benefit from DHCP because of the lower administrative burden. Most small companies cannot afford a full time network administrator who knows the ins and outs of IP addressing. Typically they delegate network administration to the one person in the office who is the most computer-savvy, whether or not he or she has technical training or experience with networking. By utilizing DHCP, the day-to-day administration of IP addressing and associated configuration details is handled automatically without any intervention from office personnel.

The biggest problem for small companies is the initial implementation of DHCP. Small companies may have to use a consultant for the initial implementation, during which the designated administrator is trained in the administration of DHCP. Alternatively, the administrator can attempt the DHCP implementation through trial and error, although this is not recommended.

Larger enterprises benefit from DHCP on two fronts: lower administrative burden and standardized IP configurations throughout the organization.

The benefit of lower administrative burden is similar to the benefit reaped by a small company, with the addition of the time saved from administering an IP address table. The next section about static IP addressing discusses some of the problems with manually maintaining the IP address table.

Larger enterprises also benefit from standardized IP configurations. Using standardized configurations minimizes connectivity problems relating to incorrect IP addresses, subnet masks, and default gateways. It also diminishes name resolution errors resulting from incorrect DNS and WINS addresses.

DHCP can also benefit organizations with a mobile workforce. With valid IP addresses in short supply, assigning static addresses to users with laptops would be both inefficient and foolish. The very nature of mobile users dictates that they will be connecting to the corporate network intermittently. Thus they do not require the constant exclusive use of an IP address. By using DHCP, an administrator can configure the DHCP server to reclaim these IP addresses after a short period of time. For example, for a company with 500 mobile users and 200 valid IP addresses to allocate, the administrator can set up the DHCP server to allocate these 200 IP addresses to mobile users. The administrator configures the lease time for the mobile users scope to a short duration, say one day. When a mobile user connects to the network, the DHCP client on the user’s laptop negotiates an IP address lease from the DHCP server. The mobile user then proceeds to access network resources, such as email and file services. When the user is finished, he disconnects from the network. The DHCP server then reclaims the IP address once the one-day lease period expires.

Another option for a mobile workforce is to utilize a DHCP User Class. This is a new feature found in the Windows 2000 DHCP server. It allows you to assign additional configuration data to a particular set of users. Let’s continue the example above. Instead of configuring a separate scope for the 200 IP addresses, the administrator could create a DHCP User Class for the mobile users. The user class would specify a lease period that is shorter in duration than the rest of the scope. The administrator would then configure each laptop’s DHCP client to specify that the laptop is a member of this user class.