70-291: MCSE Guide to Managing a Microsoft Windows

Server 2003 Network

Chapter 4: Dynamic Host Configuration Protocol

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Objectives

• Outline the benefits of using DHCP• Describe the DHCP lease and renewal process• Install and authorize the DHCP service• Configure DHCP scopes• Create DHCP reservations for client computers• Configure DHCP options• Understand and describe the purpose of a DHCP

relay• Install and configure a DHCP relay

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DHCP Overview

• Used to automatically deliver IP addressing • Reduces the amount of time you spend configuring

computers on your network• Used by default unless you specify otherwise• The ipconfig /all command will indicate whether the

configuration came from a DHCP server computer

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DHCP Overview (continued)

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DHCP Overview (continued)

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Leasing an IP Address

• An IP address is leased during the boot process• The overall process is composed of four broadcast

packets:• DHCPDISCOVER

• DHCPOFFER

• DHCPREQUEST

• DHCPACK

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Leasing an IP Address (continued)

• Any DHCP server that receives the DHCPDISCOVER packet responds with a DHCPOFFER packet

• The DHCP client responds to the DHCPOFFER packet it receives with a DHCPREQUEST packet

• A DHCPACK packet indicates confirmation that the client can use the lease

• Once DHCPACK is received, the client can start using the IP address and options in the lease

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Leasing an IP Address (continued)

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Renewing an IP Address

• The IP address can either be permanent or timed• A permanent address is never reused for another

client• Timed leases expire after a certain amount of time• Windows clients attempt to renew their lease after

50% of the lease time has expired• A DHCP server may either honor or reject a renew

request

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Renewing an IP Address (continued)

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Installing and Authorizing the DHCP Service

• A DHCP service must be authorized after installation

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Installing the DHCP Service

• DHCP is a standard service • It is included in Windows Server 2003• It is not installed as part of a default installation

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Installing the DHCP Service (continued)

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Authorizing the DHCP Service

• Unauthorized DHCP servers can hand out bad information

• DHCP will not start unless authorized• If Active Directory is used, authorization takes place

in Active Directory• DHCP servers are automatically authorized under

certain conditions

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Authorizing the DHCP Service (continued)

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Authorizing the DHCP Service (continued)

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Configuring DHCP Scopes

• Scope defines a range of IP addresses • Each scope is configured with:

• Description• Starting IP address• Ending IP address• Subnet mask• Exclusions• Lease duration

• Two strategies exist for defining the starting and ending IP addresses• Allow all and exempt the few static addresses• Use only the addresses not already in use

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Configuring DHCP Scopes (continued)

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Configuring DHCP Scopes (continued)

• Exclusions are used to prevent some IP addresses from being handed out dynamically

• Lease duration defines how long client computers are allowed to use an IP address

• Default lease duration is eight days• A scope must be activated before the DHCP service

can begin using it

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Superscopes

• Used to combine multiple scopes into a single logical scope

• Allows multiple scopes to be treated as a single scope• If a superscope is used, then the DHCP server offers

only one lease as opposed to multiple leases

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Superscopes (continued)

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Multicast Scopes

• Used to deliver multicast addresses to applications that require it

• Multicast addresses are used to deliver packets to groups of computers

• Start and end IP addresses define the range of addresses that can be handed out by DHCP servers

• TTL defines the number of routers through which a multicast packet can move

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Multicast Scopes (continued)

• Exclusions define addresses that should not be handed out

• Lease duration defines the length of time that an application can use a multicast address

• Default lease length is 30 days

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Creating DHCP Reservations

• Reservations are used to hand out a specific IP address to a particular client

• Useful when delivering IP addresses to devices that would normally use static addresses

• Can also be beneficial when firewalls are in place• Reservations are created based on MAC addresses

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Creating DHCP Reservations (continued)

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Configuring DHCP Options

• DHCP can hand out a variety of other IP configuration options

• It is common that all workstations within an entire organization use the same DNS servers

• DNS is often configured at the server level

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Configuring DHCP Options (continued)

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Configuring DHCP Options (continued)

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Vendor and User Classes

• Used to differentiate between clients within a scope • Vendor classes are based on the operating system• User classes are defined based on network

connectivity or the administrator• You can use the ipconfig /setclassid command to set

the DHCP user class ID

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Vendor and User Classes (continued)

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Vendor and User Classes (continued)

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Configuring a DHCP Relay

• DHCP packets cannot travel across a router• A relay agent is necessary in order to have a single

DHCP server handle all leases• Relay agents receive broadcast DHCP packets and

forward them as unicast packets to a DHCP server• The DHCP relay cannot be installed on the same

server as the DHCP service

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Configuring a DHCP Relay (continued)

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Configuring a DHCP Relay (continued)

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Summary

• DHCP dynamically assigns IP address information to clients on a network

• The DHCP lease process is composed of four packets: • DHCPDISCOVER• DHCPOFFER• DHCPREQUEST• DHCPACK

• A DHCP client attempts to renew its lease at 50%, 87.5%, and 100% of the lease time

• The commands ipconfig /release and ipconfig /renew can be used to release and renew DHCP leases

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Summary (continued)

• If the Active Directory service is present on your network, each DHCP server must be authorized in Active Directory to lease addresses to clients

• A scope defines a range of IP addresses that are leased to clients

• A superscope combines two scopes into a single logical unit to service network segments with two subnets

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Summary (continued)

• An exclusion in a scope can stop a DHCP server from handing out specific addresses

• A reservation allows you to give a specific workstation a defined IP address by tying the DHCP lease to the MAC address of the client

• Vendor and user classes can be used to configure some client computers with different options, depending on the class to which they belong

• A DHCP relay agent is required on each network that requires IP configuration from a DHCP server across a router