DHCP (DYNAMIC HOST CONFIGURATION PROTOCOL)

DHCP stands for Dynamic Host Configuration Protocol. DHCP is used to control the network configuration of a host through a remote server. It comes installed as a default feature in most of the contemporary operating systems. DHCP is an excellent alternative to the time-consuming manual configuration of network settings on a host or a network device.

DHCP works on a client-server model. Being a protocol, it has its own set of messages that are exchanged between client and server.


  • A client boots and initiates its network hardware
  • client sends out a DHCPDISCOVER message :
    • Source MAC  is – Client’s MAC
    • Destination MAC is – Hardware-layer broadcast (FFFFFF-FFFFFF) all 1’s
    • Message type is – DHCPDISCOVER
    • Server hears the DHCPDISCOVER request and    responds
      • Source MAC is – Server’s MAC
      • Destination MAC is – Client’s MAC
      • Message type is – DHCPOFFER containing:
  1. Server-provided IP address from pool of free addresses (the server should but is not required to check for address conflicts before offering the IP address).
  2. List of DHCP configuration parameters
  • Client responds with DHCPREQUEST message and does one or more of the following:
  1. requests values for the server-offered parameters from a single server (rejecting all offers from other servers)
  2. confirm the correctness of the previously allocated IP address (after the client had rebooted or lost connection to the network)
  3. requests extension of the lease on the specific address already supplied.
  • The server responds with
  1. a DHCPACKnowledge to confirm the server-offered options and IP previously confirmed by the client –
    – or-
  2. DHCPNOACKnowledge to reject the server-offered options.
    – or –
  3. DHCPDECLINE message to indicate to the server the address is in use.
  • The client retains the information throughout the period of its lease.
  • The client sends a DHCPRELEASE message to release its IP address at the DHCP server when it is leaving the network.

 

DHCP-DIAGRAM1

 

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