A Guide to Designing a DHCP Infrastructure

DHCP can quickly become an essential piece of an organization's data network. Once set up, DHCP (Dynamic Host Configuration Protocol) is usually hardly noticed, silently and faithfully performing its duties day in and day out. Unfortunately, the hardest thing about DHCP is getting it to that point.

This article discusses some of the reasons why 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 its existing environment is used and what types of users and workstations are being utilized (such as mobile users and network devices).

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. The needs of the DHCP client must be considered, including which DHCP options are supported by the client's operating system and which options and their correspomding values need to be assigned. Finally, all of these elements are brought together to implement the DHCP scopes.

Create an IP Addressing Plan

Before deciding to implement DHCP, an administrator must first decide on an IP addressing plan. There are many different ways to create an IP addressing plan, and in some cases they may need to be combined. This is a critical step because it is the foundation to the entire DHCP infrastructure.

Static IP Addressing

In an environment that uses static IP addressing, when an administrator installs a new workstation, she looks up an available IP address and the corresponding subnet mask in the IP address table. This table may be written in a notebook or saved on a computer in a spreadsheet. Once she finds the IP address, she needs to determine the correct DNS (Domain Name System) and WINS (Windows Internet Naming Service) server addresses for the workstation to use. In addition, in a routed environment, the administrator needs to ascertain the correct default gateway address for the workstation's subnet. Then she manually configures the workstation with the proper TCP/IP information. For small networks or networks that do not experience many changes, this may be fine.

Related Reading DHCP for Windows 2000 Managing the Dynamic Host Configuration Protocol By Neall Alcott Table of Contents Index Sample Chapter Author's Article Read Online--Safari Search this book on Safari:   Only This Book All of Safari Code Fragments only

There are a number of downsides to administering IP addresses statically. Foremost is human error. If the administrator mistypes the IP address or subnet mask, the workstation may not have connectivity to the resources it requires. If the DNS or WINS server IP addresses are mistyped, the workstation will not be able to perform name resolution. If the default gateway is incorrect, the workstation will not be able to connect to remote subnets and resources.

Another downside is maintaining the IP address table. The administrator must continually spend time viewing and searching the address table for available addresses. Once she finds an available address, the administrator must note in the table that the IP address is now in use.

Finally, if the network is large and its users move about often, using static IP configurations can be frustrating and inefficient. Problems such as the ones described earlier are compounded with larger networks.

Dynamic IP Addressing

There are four methods of dynamically allocating IP addresses: automatic, dynamic, roaming, and manual. Three of these methods--dynamic, roaming, and manual--use DHCP to allocate the IP addresses.

1. Automatic allocation. Automatic IP addressing utilizes the client's operating system to allocate a private IP address. Microsoft's Windows 2000 and Windows 98, along with the Apple Mac OS 8.5 and later, are operating systems that support Automatic Private IP Addressing (APIPA).

   The theory behind APIPA is that small ad hoc networks will be able to achieve basic connectivity without the need for intervention by the administrator. APIPA allows a workstation to configure itself with an IP address in the absence of DHCP or any other IP addressing mechanism. Automatic allocation is a quick and easy solution to the IP addressing problem, but it is only useful in small networks that need basic connectivity without Internet access. Larger environments are typically subnetted to segment network traffic and increase performance. Since APIPA is limited to the 169.254/16 subnet, it cannot be utilized in those environments. The downside to using APIPA even in small networks is the difficulty it may cause in troubleshooting connectivity issues.

2. Dynamic allocation. Dynamic allocation uses DHCP as the mechanism to allocate IP addresses. The administrator assigns a range of addresses to the DHCP server. The DHCP server, in turn, assigns an IP address in the range to DHCP clients upon request. This range is known as a scope.

   The administrator, when defining a scope, also specifies the lease duration for any IP address assignments from the scope. A lease duration is the amount of time a DHCP client has exclusive use of an IP address. After the lease duration has expired, the DHCP client must request a new lease from a DHCP server.

3. Roaming allocation. Roaming allocation can be used in situations where there are areas that users may visit temporarily with their laptops. Such areas may be libraries, classrooms, laboratories, or conference rooms, where users will need a DHCP-allocated address for a brief period of time.

   The basic configuration of the roaming allocation method is much like the dynamic allocation method, with the notable exception that the lease duration time is very short for the scopes that service these areas. For example, a company may have a conference room where users want to utilize network resources via their laptops. For the roaming allocation method to work, the conference room LAN first needs to be segmented. This is required because a subnet can be serviced by only one scope at a time. The administrator then creates a scope for the conference room subnet. The scope is given a lease duration of about 45 minutes. When users connect to the conference room LAN, they receive an IP address from the conference room scope. Once they leave the conference room, the user can wait for the lease to expire, at which point the laptop will restart the DHCP conversation. They could also release the IP address and request a new one. The roaming allocation method is useful in small, local implementations. Although it can be used on a larger scale, the short lease duration may cause excessive DHCP traffic and additional load on the DHCP servers.

4. Manual allocation. Manual allocation is another method that can be used in situations where an administrator wants to know the MAC (Media Access Control) address of the DHCP client before assigning an IP address. An administrator may want to do this for security reasons, or she may simply want to know who is utilizing network resources for billing purposes.

   Manual allocation is typically used in academic settings. Manual allocation can also be used for network devices such as servers and network printers. In this case, the MAC address of the server is used to create a reservation. With reservations, changes can be made to the IP configurations of all servers in a particular scope or even the entire enterprise. For example, if an administrator wants all servers to point to another DNS server, she could simply change the Name Server option for the scope where the servers were located. When a server renews its address lease, it will receive the updated Name Server option.

   As you can see, manual allocation is a very time consuming and labor intensive. In essence, manual allocation is very similar to using BOOTP. It should be used only in environments that require knowledge of what devices are connecting to the network.

Examine the Network Topology

When designing a DHCP infrastructure, it is important to take into account the topology of the network being serviced. By determining the topology, the designer will be able to anticipate where the load on the DHCP servers may be high and identify single points of failure that may cause DHCP services to be disrupted.

There are two different areas that need to be examined:

• The physical layout of the network
• The number of users in each physical location

By determining the physical layout of the network, the designer will be able to create a list of subnets that need to be serviced by DHCP. This information will be needed when scopes are created later. Another important factor is the placement of DHCP Relay Agents. The physical layout of the network establishes which routers and subnets will need to be serviced by relay agents.

The number of users in each location helps determine the placement of DHCP servers. If there are a small number of users located in a single location, the DHCP server may be placed in a remote subnet with a DHCP Relay Agent set up on the router to listen for DHCP requests. This eliminates the need to place a server physically on the LAN where the users reside. If the WAN link goes down, the number of users disrupted is minimized. In situations where the number of users is high, the DHCP server should be placed locally. In this case the loss of the WAN link will not disrupt DHCP service.

Define DHCP Client Requirements

Before creating any scopes, an administrator must first determine the needs of the DHCP clients the scope will be servicing. Besides receiving an IP address, subnet mask, and default gateway from a DHCP server, DHCP clients can receive DHCP options that supply many different configuration parameters.

Deciding which DHCP options to include can be determined by asking the following questions:

• Which DHCP options do DHCP clients in this scope require?
• What DHCP clients are in use on the network?
• Which DHCP options do the DHCP clients support?

Determining which options are required is relatively simple, unless there are applications in use that have special needs. Besides determining which options to use, an administrator must determine the values of those options as well.

Next, an administrator must determine which DHCP clients are in use on the network. Since Microsoft operating systems are the most prevalent on most corporate desktops and laptops, it can pretty much be said that almost every network includes some Microsoft DHCP clients. But there may be other types of DHCP clients as well, such as Unix, Linux, or Macintosh. Although these operating systems can all be DHCP clients, their implementations of DHCP vary.

Define DHCP Scopes

Now that the IP addressing plan, network topology, and DHCP client needs have been defined, it is time to start defining the various scopes.

Address Ranges

When defining a scope, the most important information to define is the address range of the scope. The address range will be used by the DHCP server to determine which IP address to assign to a DHCP client. The address range is defined by the subnet the scope will be servicing. For example, if the subnet is 10.64.0.0/11, the valid range of IP addresses for this scope is 10.64.0.1 through 10.95.255.254.

For any statically configured network devices on that subnet, exemptions have to be created. An exemption designates an IP address not to be assigned to a DHCP client. If a static IP address was not exempted, the DHCP server may assign the IP address to a DHCP client. As a result, an IP address conflict could occur and cause connectivity problems for the two computers involved.

Lease Durations

Lease durations determine when the DHCP server can reclaim the allocated IP address. Usually the default period, eight days, is more than sufficient for most scopes. Setting the lease duration too long will cause IP addresses to be shown as allocated, thus unable to be reclaimed. Setting the lease duration too short may cause excessive DHCP traffic on the network as DHCP clients attempt to renew their address leases.

Options

Any options required by the DHCP clients being serviced by the scope need to be configured at this point. Options such as the Router Option (3) need to specify the IP address for the default gateway on the subnet. Other options should be specified as well (for example, the IP addresses of the DNS servers that will be servicing the subnet).

Summary

There are many different components that need to come together to create a sound DHCP design. DHCP can be designed cafeteria-style, implementing certain components while disregarding others. Designing DHCP in this way assures that the needs of the organization are met.

Neall Alcott has been designing, building, and managing networks for the past eight years. He has been a Microsoft Certified Trainer for four years. He is a Senior Systems Engineer for Visalign LLC, a firm specializing in Web solutions, Web infrastructure, and managed services for enterprise clients. Neall is an eBusiness Infrastructure Architect, planning and designing n-Tier server architectures using Microsoft's .NET platform. Neall resides in Deptford, New Jersey with his wife, Ginny, and their daughters, Lauren and Lindsey.

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