DHCPv4 Message Format

In this article we will discuss DHCPv4 Message Format, will make brief discussion on DHCPv4 Message Format, In last article we discuss about DHCPv4 Operation.

The DHCPv4 message format is used for all DHCPv4 transactions. DHCPv4 messages are encapsulated within the UDP transport protocol. DHCPv4 messages sent from the client use UDP source port 68 and destination port 67.

DHCPv4 messages sent from the server to the client use UDP source port 67 and destination port 68. The fields are as follows:

  • Operation (OP) Code – Specifies the general type of message. A value of 1 indicates a request message; a value of 2 is a reply message.
  • Hardware Type – Identifies the type of hardware used in the network. For example, 1 is Ethernet, 15 is Frame Relay, and 20 is a serial line. These are the same codes used in ARP messages.
  • Hardware Address Length – Specifies the length of the address.
  • Hops – Controls the forwarding of messages. Set to 0 by a client before transmitting a request.
  • Transaction Identifier – Used by the client to match the request with replies received from DHCPv4 servers.
  • Seconds – Identifies the number of seconds elapsed since a client began attempting to acquire or renew a lease. Used by DHCPv4 servers to prioritize replies when multiple client requests are outstanding.
  • Flags – Used by a client that does not know its IPv4 address when it sends a request. Only one of the 16 bits is used, which is the broadcast flag. A value of 1 in this field tells the DHCPv4 server or relay agent receiving the request that the reply should be sent as a broadcast.
  • Client IP Address – Used by a client during lease renewal when the address of the client is valid and usable, not during the process of acquiring an address. The client puts its own IPv4 address in this field if and only if it has a valid IPv4 address while in the bound state; otherwise, it sets the field to 0.
  • Your IP Address – Used by the server to assign an IPv4 address to the client.
  • Server IP Address – Used by the server to identify the address of the server that the client should use for the next step in the bootstrap process, which may or may not be the server sending this reply. The sending server always includes its own IPv4 address in a special field called the Server Identifier DHCPv4 option.
  • Gateway IP Address – Routes DHCPv4 messages when DHCPv4 relay agents are involved. The gateway address facilitates communications of DHCPv4 requests and replies between the client and a server that are on different subnets or networks.
  • Client Hardware Address – Specifies the physical layer of the client.
  • Server Name – Used by the server sending a DHCPOFFER or DHCPACK message. The server may optionally put its name in this field. This can be a simple text nickname or a DNS domain name, such as dhcpserver.netacad.net.
  • Boot Filename – Optionally used by a client to request a particular type of boot file in a DHCPDISCOVER message. Used by a server in a DHCPOFFER to fully specify a boot file directory and filename.
  • DHCP Options – Holds DHCP options, including several parameters required for basic DHCP operation. This field is variable in length. Both client and server may use this field.

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