DHCP
 Dynamic Host Configuration Protocol
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Objectives
 Describe the DHCP lease and renewal process
 Understand and describe the purpose of a DHCP
relay
 Install DHCP
 Configure DHCP scopes, superscopes,
reservations, vendor classes, and user classes
 Manage and monitor DHCP
 Troubleshoot DHCP
 Install and configure a DHCP relay
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The DHCP Process
 Used to automatically deliver IP addressing
information to client computers on a network
 Can also deliver IP address information to servers
and other devices such as printers
 Use of DHCP reduces time spent configuring
computers on network
 Client computers use DHCP by default unless
static IP address is specified during installation
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Windows XP TCP/IP Properties
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 Processan
to lease
an address is composed of four
Leasing
IP Address
packets
 DHCPDISCOVER: sent from the client computer to
the broadcast IP address 255.255.255.255
 DHCPOFFER: response sent after receiving
DHCPDISCOVER packet
 DHCPREQUEST: response of DHCP client after
receiving DHCPOFFER packet
 DHCPACK: response sent by chosen DHCP server
indicating confirmation that lease has been chosen
and client can now use the lease
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The Four Packets in the DHCP Lease
Process
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Renewing
an leased
IP Address
 An IP address
using DHCP can be either
permanent or timed
 Permanent address
 DHCP server never reuses the address for another
client
 Timed lease
 Allows clients to use an IP address for a specified
period of time
 Windows clients attempt to renew their lease after
50% of lease time has expired
 ipconfig /release command is used to force the
release of a DHCP address
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The DHCP Lease Renewal Process
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 DHCP
packets
DHCP
Relay
 Are broadcast packets during the leasing process
 Cannot travel across a router
 DHCP relay
 Receives broadcast DHCP packets from clients and
forwards them as unicast packets to a DHCP server
 Must be configured with IP address of the DHCP
server to deliver unicast packets
 DHCP Relay Service cannot be installed on the
same server as the DHCP Service
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Installing DHCP
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Authorization
 Control over DHCP is very important
 An unauthorized DHCP server can quickly hand
out incorrect IP addressing information to
hundreds of client computers
 To exercise control over DHCP
 Windows Server 2003 must be authorized to start
DHCP Service
 Authorization of a DHCP server takes place in
Active Directory
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Authorization (Continued)
 To authorize DHCP server
 Must be a member of Enterprise Admins group or
 Member of Enterprise Admins group must delegate
permissions to you
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Unauthorized DHCP server error in Event
Viewer
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The DHCP Management Snap-In
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Authorized DHCP server information in
Event Viewer
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Configuring DHCP
 Normally accomplished with the DHCP
management snap-in
 NETSH
 Command used to configure DHCP
 Used in larger organizations where there is a need
to make changes programmatically using batch files
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Configuring DHCP (Continued)
 DHCP elements that can be configured include
 Scopes
 Superscopes
 Multicast scopes
 Reservations
 Vendor and user classes
 Scope, server, and reservation options
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 Used to define a range of IP addresses for the
Scopes
DHCP server to hand out to client computers
 Each scope is configured with
 Name
 Description
 Starting IP address
 Ending IP address
 Subnet mask
 Exclusions
 Lease duration
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 Name and description
Scopes
(Continued)
 Appears in the DHCP management snap-in
 Starting and ending IP addresses
 Define range of IP addresses that can be handed out
by the DHCP server
 Strategies when defining starting and ending IP
addresses
 Configure scope to use all available addresses on a
subnet, then exclude the static IP addresses being
used by hosts
 Configure scope to use addresses that are not
already in use
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 Exclusions
Scopes
(Continued)
 Used to prevent some IP addresses in a scope from
being handed out dynamically
 Lease duration
 Defines how long client computers are allowed to
use an IP address
 Default lease duration used by Windows Server
2003 is eight days
 DHCP server
 Does not begin using a scope immediately after
creation
 Scope must be activated before DHCP Service can
begin using the scope
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Scope Settings
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Superscopes
 Used to combine multiple scopes into a single
logical scope
 Used when a single physical part of the network
has two subnets
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A Superscope Containing Two Scopes
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Multicast Scopes
 Used to deliver multicast addresses to applications
that require it
 Time To Live (TTL)
 Defines the number of routers through which a
multicast packet can move
 Exclusions
 Define addresses between the start and end IP
addresses that are not handed out
 Lease duration
 The length of time that an application can use a
multicast address
 Default lease length is 30 days
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Reservations
 Used to hand out a specific IP address to a
particular client computer or device on the
network
 Can also be beneficial when firewalls are in place
 Created based on the MAC address of the network
card
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Creating a Reservation
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Configuring Options
 DHCP can hand out the following IP configuration
options
 Default gateway
 DNS server
 WINS server
 DNS is often configured at the server level
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Summary
 DHCP
 Dynamically assigns IP addresses
 Can assign multicast IP addresses
 DHCP lease process
 Composed of DHCPDISCOVER, DHCPOFFER,
DHCPREQUEST, and DHCPACK
 DHCPNAK: used by DHCP servers to decline
renewal of lease
 DHCPRELEASE: used by clients to inform DHCP
server that lease is no longer required
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Summary (Continued)
 Renewing lease
 Clients attempt to renew at 50%, 87.5%, and 100%
of lease time
 Commands ipconfig /release and ipconfig /renew
 Can release and renew DHCP leases
 DHCP server
 Must be authorized in Active Directory to lease
addresses
 Must be member of Enterprise Admins to authorize
DHCP
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Summary (Continued)
 Scope
 Defines range of IP addresses that are leased to
clients
 Must be activated before DHCP server leases
addresses in the scope
 Superscope
 Combines two scopes into single scope
 Exclusion in scope
 Used to stop a DHCP server from handing out
specific addresses or range of addresses within a
scope
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