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					HTWK 211D
 Lecture 5.1


DHCP Overview
DHCP purpose and goals
Operational details
Configuring DHCP
          What is DHCP

Dynamic Host Configuration Protocol
An extension of BOOTP (Bootstrap Protocol)
Enable client to start and automatically
configure TCP/IP
Centralize and manage TCP/IP information
                       Why BOOTP?
Bootstrap Protocol
  Provision of IP configuration for hosts
     1.    What’s my IP?
     2.    What’s my subnet mask?
     3.    Who’s my router?
     4.    Who’s my DNS?
                                                          DNS         File Server
    ee:34:d6:   e3:23:d2:7    e6:34:d6:34   e6:75:e2:6
    75:03:e2    5:05:f1       :05:44        4:66:38

                                   Boot Server      Config
        BOOTP Limitations
Designed for static environment
Doesn’t dynamically provide IP addresses
Doesn’t easily accommodate laptops or wireless
Requires manual intervention by System Administrator

Dynamic Host Configuration Protocol (DHCP),
the new protocol extends BOOTP in two ways.
  First, DHCP allows a computer to acquire all the
  configuration information it needs in a single
  Second, DHCP allows a computer to obtain an IP
  address quickly and dynamically.
To use DHCP dynamic address allocation
mechanism, a manager must configure a
DHCP server by supplying a set of IP
addresses (scope).

Whenever a new computer/client connects to
the network, the new computer contacts the
server and requests an address.
The server chooses one of the addresses the
manager specified, and leases that address to
the computer/client.
          DHCP Operation

Client/server model
Basic address
assignment (lease
process) takes 5 steps.
DHCP Lease Process
     DHCP Lease Process

The DHCP client requests an IP address by
broadcasting a DHCPDiscover message to
the local subnet. IP address
Each DHCP server responds with a
DHCPOffer message containing an IP
address and configuration information for
lease to the client.
     DHCP Lease Process
The client indicates acceptance of the offer
by selecting the offered address and
replying to the server with a DHCPRequest
Selected DHCP server sends a DHCPAck
message, approving the lease. Other DHCP
option information might be included in the
Once the client receives acknowledgment,
it configures its TCP/IP properties using
any DHCP option information in the reply,
and joins the network.
     DHCP Lease Process

In rare cases, a DHCP server might return a
negative acknowledgment to the client.
This can happen if a client requests an
invalid or duplicate address. If a client
receives a negative acknowledgment
(DHCPNak), the client must begin the
entire lease process again.
           Lease Renewal
After about 50% of the lease time has elapsed,
the client sends a DHCPREQUEST message to
renew the lease
   If server responds with DHCPACK, the lease
   and configuration parameters are updated
   If server responds with DHCPNAK message,
   client must give up address and start over at
   Step 1
           Lease Renewal
If the client hasn’t been able to renew the
lease, it tries again at 87.5% of the lease time
and broadcasts a DHCPREQUEST to all
    Any DHCP server can now return a
    DHCPACK containing the extended lease
    and updated parameters
Typical IP Configuration Details
          Benefits of DHCP
Safe and reliable configuration. DHCP minimizes
configuration errors caused by manual IP address
configuration, such as typographical errors, as
well as address conflicts caused by a currently
assigned IP address accidentally being reissued
to another computer.
Reduced network administration.
  TCP/IP configuration is centralized and automated.
  Network administrators can centrally define global and
  subnet-specific TCP/IP configurations.
  Clients can be automatically assigned a full range of
  additional TCP/IP configuration values by using DHCP
       Benefits of DHCP
Address changes for client configurations that
must be updated frequently, such as remote
access clients that move around constantly,
can be made efficiently and automatically when
the client restarts in its new location.
Most routers can forward DHCP configuration
requests, eliminating the requirement of setting
up a DHCP server on every subnet, unless
there is another reason to do so.
           Design Goals

Eliminate manual configuration
Prevent use of any IP address by more than
one host
Should not require a server on every subnet
Allow for multiple servers
      What does DHCP do?
Eliminates need for individual, manual configuration
for hosts
Includes administrative controls for network
Can coexist with hosts that have pre-assigned IP
addresses and hosts that do not participate in DHCP
     What doesn’t DHCP do

Support multiple addresses per interface
Inform running host that parameters have
Propagate new addresses to DNS
Configure routers and other network equipment
Determine other configuration parameters
Locate other servers
         Useful Resource

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