Download Module 1: Allocating IP Addressing by using Dynamic Host

Allocating IP Addressing by
Using Dynamic Host
Configuration Protocol
Overview
The Role of DHCP in the Network Infrastructure
Adding and Authorizing the DHCP Server Service
Configuring a DHCP Scope
Configuring DHCP Reservations and Options
Configuring a DHCP Relay Agent
Dynamic Assignment of IP addresses
Dynamic assignment of IP addresses is desirable for
several reasons:
 IP addresses are assigned on-demand
 Avoid manual IP configuration
 Support mobility of laptops
3
Solutions for dynamic assignment of IP addresses
Reverse Address Resolution Protocol (RARP)
 Works similar to ARP
 Broadcast a request for the IP address associated with a
given MAC address
 RARP server responds with an IP address
 Only assigns IP address (not the default router and
subnetmask)
IP address
(32 bit)
ARP
RARP
Ethernet MAC
address
(48 bit)
4
BOOTP
BOOTstrap Protocol (BOOTP)
From 1985
Host can configure its IP parameters at boot time.
3 services.
IP address assignment.
Detection of the IP address for a serving machine.
The name of a file to be loaded and executed by the client machine
(boot file name)
 Not only assign IP address, but also default router, network mask,
etc.
 Sent as UDP messages (UDP Port 67 (server) and 68 (host))
 Use limited broadcast address (255.255.255.255):
These addresses are never forwarded
5
DHCP
Dynamic Host Configuration Protocol (DHCP)
 From 1993
 An extension of BOOTP, very similar to DHCP
 Same port numbers as BOOTP
 Extensions:
Supports temporary allocation (“leases”) of IP addresses
DHCP client can acquire all IP configuration parameters needed to
operate
 DHCP is the preferred mechanism for dynamic
assignment of IP addresses
 DHCP can interoperate with BOOTP clients.
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BOOTP Interaction
(b)
(a)
Argon
00:a0:24:71:e4:44
BOOTP Server
Argon
128.143.137.144
00:a0:24:71:e4:44
DHCP Server
BOOTP Response:
IP address: 128.143.137.144
Server IP address: 128.143.137.100
Boot file name: filename
BOOTP Request
00:a0:24:71:e4:44
Sent to 255.255.255.255
(c)
BOOTP can be used for
downloading memory image for
diskless workstations
Assignment of IP addresses to
hosts is static
7
DHCP Interaction (simplified)
Argon
128.143.137.144
00:a0:24:71:e4:44
DHCP Server
DHCP Response:
IP address: 128.143.137.144
Default gateway: 128.143.137.1
Netmask: 255.255.0.0
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BOOTP/DHCP Message Format
OpCode
Hardware Type
Number of Seconds
Hardware Address
Hop Count
Length
Unused (in BOOTP)
Flags (in DHCP)
Transaction ID
Client IP address
Your IP address
Server IP address
Gateway IP address
Client hardware address (16 bytes)
Server host name (64 bytes)
Boot file name (128 bytes)
Options
(There are >100 different options)
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BOOTP/DHCP
OpCode: 1 (Request), 2(Reply)
Note: DHCP message type is sent in an option
Hardware Type: 1 (for Ethernet)
Hardware address length: 6 (for Ethernet)
Hop count: set to 0 by client
Transaction ID: Integer (used to match reply to
response)
Seconds: number of seconds since the client started to
boot
Client IP address, Your IP address, server IP address,
Gateway IP address, client hardware address, server
host name, boot file name:
client fills in the information that it has, leaves rest blank
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Lesson: Adding and Authorizing the DHCP
Server Service
Why Use DHCP?
What Is Automatic Private IP Addressing?
How DHCP Allocates IP Addresses
How the DHCP Lease Generation Process Works
How the DHCP Lease Renewal Process Works
How a DHCP Server Service Is Authorized
Practice: Adding and Authorizing a DHCP Server Service
Why Use DHCP?
DHCP reduces the complexity and amount of administrative work by
using automatic TCP/IP configuration
Manual TCP/IP Configuration
Automatic TCP/IP Configuration
IP addresses are entered
manually
IP addresses are supplied
automatically
IP address could be entered
incorrectly
Correct configuration information
is ensured
Communication and network
issues can result
Client configuration is updated
automatically
Frequent computer moves
increase administrative effort
A common source of network
problems is eliminated
What Is Automatic Private IP Addressing?
APIPA automatically self-configures addresses when there is no
DHCP server available
Advantages
Disadvantages
Serves as a DHCP server failover
mechanism for small networks
Forces assignment of addresses
typically not used
Automatically assigns an IP
address in a specific range
Conceals possible connectivity
problems
Does not work outside 169.254.x.x
subnet
Is not routable
How DHCP Allocates IP Addresses
DHCP Client2:
IP configuration
from DHCP server
Non-DHCP Client:
Static IP
configuration
Lease Renewal
Lease Generation
DHCP Server
DHCP Client1:
IP configuration
from DHCP server
DHCP
Database
IP Address1: Leased to DHCP Client1
IP Address2: Leased to DHCP Client2
IP Address3: Available to be leased
How the DHCP Lease Generation Process Works
DHCP
Server2
DHCP
Server1
DHCP
Client
1
DHCP client broadcasts a DHCPDISCOVER packet
2
DHCP servers broadcast a DHCPOFFER packet
3
DHCP client broadcasts a DHCPREQUEST packet
4
DHCP Server1 broadcasts a DHCPACK packet
How the DHCP Lease Renewal Process Works
DHCP
Server2
DHCP
Server1
DHCP Client
50%
100%
of
lease
87.5%
50% of
of
oflease
lease
lease
duration
duration has
has
expired
expired
DHCPfails
Client
sends
DHCPREQUEST
packet
client
packet
If 1
the client
tosends
renewaait’s
itsDHCPREQUEST
lease,
lease, after
after 50%
87.5%
of of
thethe
lease
lease has
duration then
expired,
has expired,
the DHCP
then
lease
thegeneration
DHCP leaseprocess
renewalstarts
process
overwill
Server1
sends
a DHCPACK
begin
again
with
againa after
DHCP87.5%
client
ofbroadcasting
the
lease duration
apacket
DHCPDISCOVER
has expired
2 DHCP
How a DHCP Server Service Is Authorized
Domain
Controller
If DHCP
Server1
findswith
its IP
DHCP
Server1
checks
the
addresscontroller
on the list,
service
domain
to the
obtain
a list
starts
and supports
DHCP
clients
of authorized
DHCP
servers
DHCP Server1
Authorized
Active
Directory
Services DHCP
requests
DHCP Server2
DHCP Client
Unauthorized
DHCPServer2
Server2 does
checks
If DHCP
notwith
findthe
its IP
DHCP
client
receives
IP
address
domain controller
obtain
a list
of
address
on the list,tothe
service
does
from
authorized
DHCP
Server1
authorized
DHCPDHCP
servers
not start
and support
clients
Does not service
DHCP requests
DHCP authorization is the process of registering the DHCP Server service in the
Active Directory domain to support DHCP clients
What Are DHCP Scopes?
A scope is a range of IP addresses that are available to be leased
DHCP Server
LAN A
LAN B
Scope A
Scope B
Scope Properties
Network ID
Lease duration
Scope name
Subnet mask
Network IP
address range
Exclusion range
What Is Network Monitor?
Network Monitor:
2
3
Captures a sample of
network traffic
Uses filters to select
specific packets
Decodes the packets
1
4
Compiles network
statistics
What Is a DHCP Reservation?
A reservation is a specific IP address, within a scope, that is
permanently reserved for lease to a specific DHCP client
Workstation 1
Subnet A
File and
Print Server
Subnet B
DHCP Server
IP Address1: Leased to Workstation 1
IP Address2: Leased to Workstation 2
IP Address3: Reserved for File and Print Server
Workstation 2
What Are DHCP Options?
DHCP options are configuration parameters that a DHCP
server assigns to clients
DHCP Client
DHCP Client IP Configuration Data
Client’s IP address
Client’s subnet mask
DHCP options
DHCP Server
How DHCP-Server, Scope, and Reserved-Client
Options Are Applied
DHCP Server
File and
Print Server
Windows 98
Scope A
Scope B
Router
Windows XP
Windows XP
DHCP option applied at the
reserved-client
server
scope levellevel
How DHCP Class-Level Options Are Applied
DHCP Server
File and
Print Server
Windows 98
Router
Router
Scope A
Scope B
Windows XP
Windows XP
DHCP option applied at the
class level
Lesson: Configuring a DHCP Relay Agent
What Is a DHCP Relay Agent?
How a DHCP Relay Agent Works
How a DHCP Relay Agent Uses Hop Count
How a DHCP Relay Agent Uses Boot Threshold
Practice: Configuring a DHCP Relay Agent
What Is a DHCP Relay Agent?
A DHCP relay agent is a computer or router that listens for DHCP/BOOTP
broadcasts from DHCP clients and then relays those messages
DHCP Relay Agent
DHCP Server
Unicast
Broadcast
Broadcast
Subnet A
Subnet B
Routers
(Non–RFC 1542
Compliant)
Client
Client
Client
Client
How a DHCP Relay Agent Works
DHCP Relay Agent
Client1
1
2
3
4
5
6
7
8
Client2
DHCP Server
Router
Non-RFC 1542 Compliant
(Non–RFC
Compliant)
Client3
Client1 broadcasts a DHCPDISCOVER packet
Relay agent forwards the DHCPDISCOVER message to the DHCP server
Server sends a DHCPOFFER message to the DHCP relay agent
Relay agent broadcasts the DHCPOFFER packet
Client1 broadcasts a DHCPREQUEST packet
Relay agent forwards the DHCPREQUEST message to the DHCP server
Server sends a DHCPACK message to the DHCP relay agent
Relay agent broadcasts the DHCPACK packet
How a DHCP Relay Agent Uses Hop Count
The hop count threshold is the number of routers through which the
packet can be transmitted before it is discarded
DHCP Relay Agent 2
Hop Count = 2
DHCP Relay Agent 1
DHCP Server
How a DHCP Relay Agent Uses Boot Threshold
The boot threshold is the time the DHCP relay agent will wait for a
DHCP server response before forwarding the request
DHCP Server 2
Boot Threshold
= 10 seconds
DHCP Relay Agent
Local DHCP
Server
DHCP Server 3