Dynamic Host Configuration Protocol(DHCP)

Objectives

• What is 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

What is DHCP?

• It stands for Dynamic Host Configuration Protocol

• DHCP automatically assign the IP address to the

computer

• The ipconfig /all command will indicate whether the

configuration came from a DHCP server computer

How to assign IP address to a computer?

Leasing an IP Address

• An IP address is leased during the boot process

• Default lease period is 8 days

• Maximum lease period is 999 days

• Leasing an IP address is performed by DORA

process

DORA Process

• DORA process means

▫DHCP DISCOVER

▫DHCP OFFER

▫DHCP REQUEST

▫DHCP ACK

Simple network

7

Hello Mr. Server, I need to connect to the Internet, could you be kind to hook me up with an IP address?Ok dude, I got some in

my pool of address, I lend you it. Check it out?Your IP : 123.45.78.6

Thank you Mr. Server, I like your offer. I will use it.

You’re welcome, here is your configuration but it’s only for 3 days.

DHCP server discover

DHCP server offer

DHCP request

DHCP acknowledge

Router

Internet

DHCP Server

UDP Port 68

UDP Port 68

UDP Port 68

UDP Port 67

DHCP Clients

DHCP protocol is a mainly 4-step process:

DHCP ServerDHCP Clients

Broadcast

Unicast

DHCP Message

Use

DHCPDISCOVER Client broadcast to locate available servers

DHCPOFFER Server to client response offering configuration parameters

DHCPREQUEST Client broadcast requesting offered parameters

DHCPDECLINE Client to server notification that IP address is in use

DHCPACK Server to client response confirming a request

DHCPNAK Server to client response denying a request

DHCPRELEASE Client to server request to relinquish IP address

DHCPINFORM Client to server request for configuration parameters

DHCP Message Types

8

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

9

Renewing an IP Address (continued)

10

Installation of DHCP

• Install DHCP on Windows Server 2003

• Make sure your network connection is statically

configured

• Install the service using the Add/Remove Windows

Components utility

Installing DHCP service

Authorizing the DHCP Service

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

Creating a Scope

• Create a scope to distribute IP addresses to client

computers

• Manually enter the IP configuration settings as

directed by the text

• Create a new scope using the configuration settings

provided

Configuring DHCP Scope

• Scope is a range of IP address

• Each scope is configured with:• Description

• Starting IP address

• Ending IP address

• Subnet mask

• Exclusions Range

• Lease duration

Configuring DHCP Scope

Types of Scope

• Super Scopes

• Multicast Scopes

Super scopes

• It is a collection of individual scope• Combine two scopes into a single logical unit using a

super scopes• First, create a second scope in addition to the scope

already created in a previous activity• Create a super scopes to encompass the two scopes• Use the DHCP snap-in for this activity

Super scopes

Multicast scopes

• It is used to deliver multicast address to a group of computers

• It is defined by using the following parameters• Starting IP address• Ending IP address• TTL• Exclusions• Lease duration

Creating DHCP Reservations• Reservations are used to hand out a specific IP

address to a particular client• Reservations are created based on MAC addresses

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

Configuring Relay agents

•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

Example with Relay Agent

Router

Internet

Relay agent

Router

67UDP

Broadcast

Source123.45.67.8Destination : 1.1.1.1Client IP : 123.45.68.7DHCPOFFER

123.45.67.8

BroadcastSource : 0.0.0.0Destination : 1.1.1.1DHCPDISCOVER

Unicast

Source : 123.45.67.8

Destination : 123.45.55.3

UnicastSource : 123.45.67.8

Destination : 123.45.55.3

DHCP Server

123.45.55.3

Unicast

Source : 123.45.67.8

Destination : 123.45.55.3Client IP: 123.45.68.7DHCPOFFER

Unicast

Source : 123.45.53

Destination : 123.45.68

Client IP : 123.45.68.7

New DHCP Client

68 UDP

25

Advantage

• It minimize the administrative burden• There is no chance to conflict IP address• It supports multiple scope like multicast scope and super

scope

Disadvantage • The machine name does not change when you get a new IP

address• When DHCP server is unavailable client unable to access

the network