and how it works
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…and how it works. DHCP. Overview. Introduction Basics Case - We play DHCP Server Problems Tasks Solutions Apply what you\'ve learned Conclusion. Basics. D ynamic H ost C onfiguration P rotocol Standard protocol Defined by RFC 1541 (superseded by RFC 2131) Created by the

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Presentation Transcript
and how it works
…and how it works


by steffen diehl

  • Introduction
  • Basics
  • Case - We play DHCP Server
    • Problems
    • Tasks
    • Solutions
    • Apply what you\'ve learned
  • Conclusion


  • Dynamic Host Configuration Protocol
  • Standard protocol
  • Defined by RFC 1541 (superseded by RFC 2131)
  • Created by the

Host Configuration Working Group

of the

Internet Engineering Task Force (IETF)

in general

  • Runs over UDP
  • Utilizing ports:
    • 67 – connections to server
    • 68 – connections to client
  • Extension of BOOTP (protocol used for simple interaction)
  • Uses client–server model


  • Got a network
  • Want to add new clients
  • DHCP-Server doesn’t work
  • How do clients get their:
    • IP Address -or-
    • network configuration parameters (Subnet Mask, Default Gateway, DNS and WINS)
  • Need replacement!!


case theory
Case - Theory

1st implementation goal of a DHCP Server

  • support of dynamic allocation of Network Addresses
    • chooses and allocates an IP address
    • gives network configuration parameters to the client

server tasks I

case theory7
Case - Theory
  • Need 2 databases
    • Address Pool is the database which holds IP addresses and other network configuration parameters
    • Binding database keeps mapping between an Ethernet address and Address Pool entries

parts of a DHCP Server

case theory8
Case - Theory
  • All interactions are initiated by a client
  • Server only replies
  • “obtain an IP address automatically” in NW options
  • configuring the host to the network is done by a simple handshake

client conditions

case in practice
Case – in practice
  • Client broadcasts DHCPDISCOVER
  • One or more servers return DHCPOFFER with available Address and NW Information
  • Clients chooses one offer that it likes best
  • broadcasts DHCPREQUEST to identify chosen Server/lease
  • DHCPREQUEST also to renew lease

steps in becoming part of the network I

case in practice10
Case - in practice
  • Server sends
      • Lease is finalized
      • Client starts using IP
      • Client resumes from DHCPDISCOVER point
  • If client doesn’t want IP - DHCPDECLINE is sent
  • DHCPRELEASE gives IP back into pool

steps in becoming part of the network II

case theory11
Case - Theory
  • Static/Manual allocation
    • Server allocates IP chosen by the Admin
    • Server configuration includes
      • IP-Address -and-
      • MAC-Address

for every client

    • DHCP only used to convey assigned address to client

types of assigning addresses I

case theory12
Case - Theory
  • Automatic allocation
    • IP address is permanently associated with a MAC address
    • till administrator intervenes the infinite lease
  • Dynamic allocation
    • Like Automatic allocation
    • Except:
      • server tracks leases
      • give IP addresses whose lease has expired to other DHCP clients

types of assigning addresses II


case theory13
Case - Theory
  • Client is responsible to renew/release IP
  • Lease timestamps:
    • Total lease duration
    • T1 (0.5 * duration_of_lease)
      • client enters the RENEWING state
      • contacts the server that originally issued network address
    • T2 (0.875 * duration_of_lease)
      • client enters the REBINDING state
      • attempts to contact any server

lease duration

case theory14
Case - Theory
  • Lease duration
    • Client holds IP when not connected
    • Clients retire
    • Servers/Databases should have constant IP´s
  • Analyze the NW
    • Often changing clients?
    • Servers/databases?
    • Enough IP Addresses available?


case theory15
Case - Theory

By using the fitting Methods…

implementation goals 2-4 of a DHCP Server…

  • server should be able to recycle the IP address when the lease period is expired
  • A network administrator should not configure each client/user interaction of each client should not be required.
  • support of static allocation and an infinite lease

…can also be realized!!!!

server tasks II

case in practice16
Case - in practice
  • Servers:
    • WEB-Server
    • Access Point
  • Lecturers via Ethernet:
    • Range from .100 to .102
  • Students via WLAN:
    • Range from .200 to .230

find a solution!!!


  • Insecurity of UDP
  • Broadcast of messages
  • Risk of wrong configuration causing
    • High traffic
    • Waste of IP Addresses
    • A lot more work than expected
  • Detailed and precise analysis before implementation needed

negative aspects

  • works well if you have to manage a lot of mobile users:
    • people with laptops working in and out of the office
    • hosts coming and going with a great amount of frequency
  • perfect when Network parameters have changed
    • only one point you have to work at
  • sharing a limited pool of IP addresses
  • Mixture of allocation types can be used

positive aspects


Thank you for your attention

…any questions??

the end

  • RFC 2131, Dynamic Host Configuration Protocol
  • RFC 2322, Management of IP numbers by peg-dhcp
  •, Internet Engineering Task Force
  •, Microsoft Knowledge Base Article - 169289