This post is in continuance to the previous post on DHCP fundamentals (Link) –

Now, we will understand DORA process in detail.

DORA process in DHCP is stands for following message flows between the client and the server.

  • Discover
  • Offer
  • Request
  • Acknowledge

Below diagram depicts the message flow between the DHCP client and the Server –understanding-dora-process-in-dhcp


Now let’s take a look what happens when these messages are exchanged between DHCP Client and Server. 2 key items should be kept in mind which are also important from interview point of view as well. These are –

  1. Network layer broadcast and
  2. Data Link Layer broadcast.


DHCP client sends out a DHCP Discover message to find out the DHCP client. DHCP discover message is a layer 2 broadcast as well as layer 3 broadcast.

Fields in DHCP Discover Message:

Src IP:

Dst IP:

Src MAC : DHCP clients MAC address


Hence from the above fields it is clear DHCP Discover message is a Network Layer and Data Link Layer Broadcast.


DHCP server receives the DHCP discover message from client and sends back the DHCP offer message with field information as below:

Src IP: DHCP Server IP Address

Dst IP:   #Still Broadcast as Client still has no IP Address#

Src MAC : MAC Address of DHCP Server

Dst MAC: DHCP clients MAC address

Hence from above field it is clear that DHCP offer message is a layer 2 unicast but still as layer 3 broadcast.


DHCP client receives the DHCP offer from DHCP server and sends back a DHCP Request message with following fields:

Src IP: #As still the IP address hasn’t been assigned to Client#

Dst IP:   #Still Broadcast as Client must have received Offer from more than one DHCP server in their domain and the DHCP client accepts the Offer that its receives the earliest and by doing a broadcast it intimates the other DHCP server to release the Offered IP address to their available pool again #

Src MAC : MAC Address of DHCP Server

Dst MAC: DHCP clients MAC address

Above fields concludes that DHCP request message is also a layer 2 unicast and a layer 3 broadcast.


Once the DHCP client sends the request to get the Offered IP address, DHCP server responds with an acknowledge message towards DHCP client with below fields:

Src IP: DHCP Server IP Address

Dst IP:

Src MAC : MAC Address of DHCP Server

Dst MAC: DHCP clients MAC address

 From above fields substantiates that DHCP Acknowledge is a layer 2 unicast but still a layer 3 broadcast.

For more details on the information you must get familiar with the DHCP header fields. Few important fields from DHCP header for our reference are as below –

Ciaddr: Client IP address.Yiaddr ‘your’ (client) IP address: Server’s response to client.

Siaddr Server IP address: Address of sending server or of the next server to use in the next Bootstrap process step.

 Giaddr: Relay agent IP address, used in booting via a relay agent.

Chaddr: Client hardware address.

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