Chapter 8, DHCP in IP Configuration Designs

|1| Chapter 8 Overview

A. Designs That Include DHCP

1. Identify the requirements and constraints for creating a DHCP design.

2. Describe the relationship between DHCP and Windows 2000.

3. Determine when it’s appropriate to use DHCP in IP configuration solutions.

4. Identify the DHCP design decisions you’ll need to make.

B. Essential DHCP Design Concepts

1. Determine where and how to use DHCP to provide automatic IP configuration.

2. Determine the DHCP scopes and scope options to include in your design.

C. Configuration Protection in DHCP Designs

1. Prevent IP configuration errors caused by unauthorized DHCP servers.

2. Prevent unauthorized users from modifying your DHCP server configuration.

D. DHCP Design Optimization

1. Select strategies to improve the availability and performance of your DHCP design.

Chapter 8, Lesson 1

Designs That Include DHCP

|2| 1. DHCP and Windows 2000

A. DHCP provides automatic IP configuration.

1. Makes it easier to implement and maintain networking services designs, and reduces the chance of IP configuration errors

2. Uses IP information from IP broadcasts

3. Consider the existing IP routing design as you implement DHCP.

B. Windows 2000 includes DHCP services.

1. DHCP Client

a. Receives IP configuration information from DHCP servers and updates the local IP configuration

b. Is also included in other operating systems, including Microsoft Windows 95, Microsoft Windows 98, Microsoft Windows Me, Microsoft Windows NT 4.0, and UNIX

2. DHCP Server

a. Provides IP configuration information to DHCP clients

b. Uses the Windows 2000 IP stack to communicate with DHCP clients, other DHCP servers, the Active Directory domain controllers, and DHCP Relay Agents

c. Manages a database that contains the status of the IP address ranges that the DHCP server manages

d. Is available in Microsoft Windows 2000 Server, Microsoft Windows 2000 Advanced Server, and Microsoft Windows 2000 Datacenter Server

e. Is not available in Microsoft Windows 2000 Professional

f. Requires you to assign a fixed IP address to the network interfaces in the computer running it

3. DHCP Relay Agent

|3| a. Is a protocol you can include in Routing and Remote Access

b. Forwards DHCP between IP routed network segments

c. Receives DHCP broadcasts and forwards the DHCP requests as unicast IP packets directed to the DHCP server

d. Eliminates DHCP broadcast traffic between network segments

e. Requires you to assign a fixed IP address to the network interfaces in the computer running it

f. Is a standard feature of Windows 2000 Server, Windows 2000 Advanced Server, and Windows 2000 Datacenter Server

g. Is not available in Windows 2000 Professional

h. Do not install DHCP Server service and DHCP Relay Agent on the same computer.

Note You should not install the DHCP Server service and DHCP Relay Agent on the same computer because the DHCP Server service and DHCP Relay Agent use the same UDP ports. The services won’t work reliably if you install them both on the same computer.

C. To create DHCP designs, you should understand

1. General IP configuration

2. General IP routing theory

3. General DHCP theory

|4| 2. DHCP Design Requirements and Constraints

A. Collect design requirements and constraints before creating your design.

1. The amount of data transmitted between existing network segments that contain the DHCP clients and DHCP server

2. The number of locations and network segments that require automatic IP configuration

3. Plans for future network growth

4. Characteristics of existing routers, including

a. Router placement

b. Router broadcast traffic forwarding

c. WAN connections used

|5| 3. DHCP Design Decisions

A. Base design decisions on your organization’s requirements and constraints.

B. Decide the following:

1. The types of client computers that the DHCP servers will support

2. The method for handling IP configuration for each network segment

Note Before designing a DHCP solution, you must determine the number of hosts, the number of subnets, and the configuration of the network.

3. The placement of DHCP servers and DHCP Relay Agents

4. The method for providing DHCP automatic IP configuration to DHCP clients

5. The method for optimizing traffic between DHCP clients and DHCP servers

|6| 4. IP Configuration Designs

A. Most IP configuration designs are for routed networks.

B. IP parameters for all computers in the organization’s private network must be properly configured.

C. Choose one of the following methods for configuring IP information for each computer:

1. Manual configuration

a. You must manually configure file servers, print servers, routers, gateways, or other IP devices that provide resources or services to client computers.

b. To save time and reduce configuration errors, use an automatic IP configuration method for computers that don’t manage network resources.

2. DHCP services provided by third-party operating systems

a. You can use DHCP services offered in other operating systems (like Novell NetWare and UNIX systems) to automatically configure client computers.

b. This method is useful if your organization has standardized on a third-party operating system that offers a DHCP server implementation.

c. DHCP services included with other operating systems do not provide all of the features offered by the DHCP services in Windows 2000.

|7| 3. DHCP services provided by Windows 2000

a. Are integrated with Microsoft operating systems, the Domain Name System (DNS), and Routing and Remote Access services in Windows 2000

Note Routing and Remote Access dynamically allocates IP addresses from a DHCP server to remote access clients. When the remote access client disconnects, Routing and Remote Access immediately returns the assigned IP address, which becomes available to other remote access clients.

Note DHCP and DNS integration allows earlier versions of Windows–based clients and other non-Microsoft clients to automatically update their records in the DNS database.

b. Provide automatic IP configuration for other operating systems

c. Prevent DHCP configuration errors from unauthorized DHCP servers

d. Provide highly available IP configurations by using DHCP and Windows clustering

e. Allow you to standardize network management by using Active Directory directory service

Note One of the advantages of the Windows 2000 DHCP service is its integration with Active Directory. It allows DHCP servers to be authorized within Active Directory.

Chapter 8, Lesson 2

Essential DHCP Design Concepts

|8| 1. Determining Which Segments Require Automatic IP Configuration

A. Use DHCP to configure IP information for any network segments that contain desktop computers.

B. Provide automatic IP configuration for all network segments except

1. Network segments that contain only computers that manage network resources

a. File servers

b. Print servers

c. Database servers

d. Web servers

2. Network backbone segments that contain only routers that connect network segments to the backbone

3. WAN segments

Note The devices listed above don’t need automatic IP configuration. They usually require a fixed IP address, and therefore shouldn’t use DHCP.

4. Screened subnet segments (also known as Demilitarized Zones, or DMZs).

|9| 2. Determining the DHCP Automatic IP Configuration Method

A. Designers need to determine which DHCP automatic IP configuration method to use for each network segment.

B. The automatic IP configuration methods are

1. Connecting a DHCP server to network segments

2. Connecting a DHCP Relay Agent to network segments

3. Enabling DHCP/Boot Protocol (BOOTP) forwarding on routers that connect to network segments

|10| C. DHCP server method

Note You should place a single DHCP server on one subnet (generally the one with the highest number of clients) and then use DHCP Relay Agents or DHCP/BOOTP forwarding on the routers.

1. Include at least one DHCP server at each location in your networking services design.

2. Each DHCP server can service more than 15,000 client computers.

3. In each location, use as few DHCP servers as practical, to reduce DHCP server administration tasks.

Note A simple, non-routed LAN needs only one DHCP server. For larger networks, multiple DHCP servers must share the IP address range by defining a scope.

4. To provide DHCP services to more than one network segment, include multiple network interface adapters in the computer that runs the DHCP Server service. This creates a multihomed DHCP server.

|11| D. DHCP Relay Agents method

Note The DHCP Relay Agent is one of the methods used in routed networks. The DHCP Relay Agent as an intermediary between DHCP clients and DHCP servers.

1. Include DHCP Relay Agents on network segments that are not directly connected to DHCP servers.

2. DHCP Relay Agents forward DHCP traffic between a network segment and a network segment that contains a DHCP server. The process is as follows:

a. The DHCP client sends a request.

Note The DHCP Relay Agent is transparent to the DHCP client.

b. The DHCP Relay Agent receives the broadcast packet request.

c. The DHCP Relay Agent converts the request from broadcast to unicast.

d. The DHCP Relay Agent forwards the request to the DHCP server.

3. A DHCP Relay Agent can service thousands of client computers.

4. To provide DHCP services to more than one network segment, include multiple network interface adapters in the computer that runs the DHCP Relay Agent. This creates a multihomed Relay Agent.

|12| E. DHCP/BOOTP forwarding on routers method

Note DHCP/BOOTP is the second way of supporting DHCP in a routed network. DHCP Relay Agent has advantages over DHCP/BOOTP. Routers with DHCP/BOOTP forwarding enabled forward the broadcast packet, creating additional broadcast traffic. If your routers already support DHCP/BOOTP, however, you should consider implementing DHCP/BOOTP to save costs.

1. Use this method on network segments that aren’t connected to a DHCP server.

2. DHCP/BOOTP forwarding on routers forwards only the DHCP broadcast traffic.

3. Routers with more than two network interfaces and with DHCP/BOOTP enabled can provide DHCP services to more than one network segment.

4. The method is useful in designs where an existing router supports DHCP/BOOTP forwarding and you don’t want to add more hardware or software.

|13| 3. Determining DHCP Scopes and DHCP Scope Options

A. Designers need to determine the DHCP scopes and DHCP scope options to include in the design.

B. A DHCP scope defines a range of IP addresses that the DHCP server manages.

Note Using multiple DHCP servers to provide IP addresses to the subnet increases DHCP service availability and reliability to the subnet and distributes the DHCP client load.

1. Create a DHCP scope for each IP address range managed by DHCP.

Note You must define a scope before DHCP clients can use the DHCP server for dynamic IP configuration.

2. Superscopes group multiple DHCP scopes to support multiple IP address ranges on the same physical network segment.

Note A superscope is a shared, distributed scope that allows multiple servers to share the available address range for a subnet.

3. Use superscopes to

a. Support network segments serviced by DHCP Relay Agents or routers with DHCP/BOOTP forwarding

b. Provide additional IP address ranges for network ranges that have used up the existing available IP address ranges

c. Combine multiple IP subnets into a single subnet without redesigning your IP addressing scheme and subnet masks

|14| 4. The DHCP IP address lease length specifies when an IP address assigned to a computer that was removed from a network segment becomes available for other computers to use.

Note Modifying the lease length is one of the primary ways to improve DHCP performance. If clients leave the network but do not release their IP addresses, those addresses are unavailable for other computers until the administrator manually releases all addresses.

a. Increasing the DHCP lease length

(1) Means that IP addresses are unavailable for longer periods after the computer is removed from the network segment

(2) Decreases DHCP traffic

(3) Should be done when computers are seldom moved from segment to segment

b. Decreasing the DHCP lease length

(1) Makes the IP address available sooner

(2) Increases DHCP traffic

Note Decreasing the lease length increases network traffic.

(3) Should be done when computers are frequently moved between network segments

Note You can configure Windows 2000 DHCP clients to automatically release their IP addresses when they shut down.

|15| C. Determine which IP addresses to exclude from the IP address range defined by the DHCP scope.

1. For each device with a manually assigned IP address within the DHCP scope, you must exclude the corresponding manually assigned IP address.

2. Exclude any manually assigned IP addresses within the DHCP scope’s IP address range, such as those for the following:

a. IP routers

b. Firewalls

c. File, print, and application servers

d. Gateways

e. Any operating system or device that DHCP cannot configure

f. Any other device with a manually assigned IP address

|16| D. Define the DHCP scope options.

1. A DHCP scope option is a specific client IP configuration parameter for

a. Routers

b. DNS servers

c. DNS domain name

d. Windows Internet Name Service (WINS) node type

e. WINS servers

2. Define the DHCP scope options that the DHCP administrator can assign.

3. Assign DHCP scope options to affect different levels of clients that the DHCP Server services.

a. Server options level: scope options apply to all scopes managed by the DHCP server

b. Scope options level: scope options apply to all DHCP clients with IP addresses within the range defined by the DHCP scope

c. Class options level: scope options apply to a specific, customizable class of DHCP clients

d. Client options level: scope options apply to individual client computers only

Note DHCP also provides predefined options that let you control which options are available through the DHCP console. Although you can make options available in this way, they are not assigned values until configured at the server, scope, or client.

Chapter 8, Lesson 3

Configuration Protection in DHCP Designs

Note The DHCP protocol is not inherently secure, and that there are a few management strategies you can use to make the service secure. One of the primary methods is to integrate the DHCP service with Active Directory.

|17| 1. Preventing Unauthorized DHCP Servers

A. Protect your network from unauthorized DHCP servers.

B. If an unauthorized DHCP server is started, it can automatically provide IP configuration to DHCP clients and thus disable communications.

C. Prevent unauthorized Windows 2000–based DHCP servers from starting by authorizing Windows 2000 DHCP servers in Active Directory.

1. The DHCP Server service automatically queries Active Directory to determine whether a DHCP server is authorized to start.

2. If the DHCP server is not authorized, the DHCP Server service writes an event in the Windows 2000 Event Log and stops.

D. To detect and prevent unauthorized DHCP servers, do the following:

1. Install one or more DHCP Server services on a domain controller or member server.

2. List authorized DHCP servers in Active Directory.

3. Use DHCP Relay Agents or enable DHCP/BOOTP forwarding on routers.

E. You can’t prevent DHCP servers from starting if they run on operating systems other than Windows 2000.

|18| 2. Preventing Unauthorized Users

A. Prevent unauthorized users from directly accessing your DHCP servers and compromising the integrity of your DHCP database.

B. Protect your DHCP servers and the database in two ways:

1. Restricting DHCP administrators

a. Grant permission to manage DHCP servers only to authorized network administrators.

b. Create a Windows 2000 group and assign the group the permissions needed to manage your organization’s DHCP servers.

c. In the Windows 2000 group, include the authorized network.

2. Isolating DHCP servers from public networks

a. DHCP servers should not exist on any network segments that public network users can access.

b. Make sure that only computers within your organization’s private network can directly communicate with your DHCP servers.

Chapter 8, Lesson 4

DHCP Design Optimization

Note To ensure availability, your DHCP solution must include a very reliable DHCP server or redundant DHCP servers. There are generally two methods for increasing availability: superscopes (shared distributed scopes) and Windows Clustering.

|19| 1. Enhancing DHCP Availability

A. Optimize your design for increased availability.

B. Base your decisions on your organization’s requirements.

C. Use the following strategies to increase availability:

1. Use Windows Clustering server clusters.

Note Windows Clustering is a method to increase the availability of a single DHCP server using a server cluster.

a. Windows 2000 DHCP Server service is cluster-aware, which means that it can interact with server clusters.

b. You can create server clusters by configuring two computers to share a common cluster drive. The DHCP database is stored on this drive.

c. The DHCP Server service runs on only one cluster node, called the active node, at a time.

d. If the DHCP active node fails for any reason, the other cluster node automatically runs the DHCP Server service. The shared drive contains the current DHCP reservation information from the DHCP database.

2. Distribute a DHCP scope across multiple DHCP servers.

a. Distributes the IP address range that the DHCP scope manages across two DHCP servers.

b. If one DHCP server fails for any reason, the other DHCP server provides IP configuration for the segment with the remaining portion of the IP address range.

c. Requires no additional hardware or software resources

Note You can also allocate equal portions of the address range if subnets use DHCP Relay Agents.

3. Dedicate a computer to DHCP to prevent one application or service from becoming unstable.

|20| 2. Improving DHCP Performance

A. Business requirements can include set periods, or a number of simultaneous DHCP connections.

B. You can improve DHCP configuration performance by

1. Load balancing the DHCP configuration across multiple DHCP servers

a. This method is useful when existing DHCP servers are working to capacity and you cannot upgrade hardware.

b. Evenly distributes DHCP scopes across multiple DHCP servers, which distributes IP configuration traffic across servers and reduces any latency in IP configuration

2. Modifying the lease length

a. Increasing the lease length reduces the IP configuration traffic but means that IP addresses are reserved for longer periods.

b. Decreasing the lease length increases the IP configuration traffic but increases the availability of IP addresses.

3. Dedicating a computer to DHCP to prevent other applications and services from consuming system resources

|21| Chapter Summary

A. DHCP services in Windows 2000 provide automatic IP configuration for the client computers, reducing errors as well as configuration and management time.

1. Manually configure IP information for IP devices that manage network resources (such as file servers, print servers, and so on).

2. Include at least one DHCP server at each geographic location in your design.

B. Use DHCP Relay Agents or enable DHCP/BOOTP forwarding on routers wherever network segments are not directly connected to DHCP servers.

C. Create DHCP scopes for all IP address ranges that are automatically configured using DHCP.

1. With Windows 2000, you can also create superscopes.

2. Exclude any manually assigned IP addresses within the scope’s IP address range.

3. You can assign DHCP scope options to affect different levels of DHCP clients that a DHCP server manages.

D. Use the DHCP Server service in Windows 2000 to prevent unauthorized DHCP servers from starting and communicating with the network.

E. Grant DHCP management permissions carefully to protect the DHCP database.

F. Use the following optimization techniques to improve the availability and performance of your DHCP design:

1. Use Windows Clustering server clusters.

2. Distribute DHCP scopes across multiple DHCP servers.

3. Dedicate a computer to DHCP.

4. Modify the DHCP lease length.