DHCP IP Allocation Operation

Table of Contents

How Does DHCP Achieve IP Allocation Operation?

DHCP does its job with Messages like many protocol. Here, as we mentined before, DHCP uses DHCP Messages. Let’s see how these DHCP Messages are used in DHCP IP Allocation Operation.

Here, firstly we will explain the main DHCP Messages for a successfulDHCP IP Allocation Operation. In this operation four DHCP Message is used.

Now, let’s see these four DHCP Messages and the other DHCP Messages one by one.

DHCP Discover Message

For IP allocation, firstly, DHCP Client sends a Broadcast DHCP Discover Message. This DHCP Discover Messege ask this question :

“Are there any DHCP Server in the Network?”DHCP Client ask this question to all the nodes. Because DHCP Discover Message is sent as Broadcast. Here, the destination port is UDP 68. In this message, there are some other required fields but the most important one is the MAC address of DHCP Client.

There is also one more important flag in DHCP Discover Message. This flag is “Broadcast Flag”. If this flag set to “1”, this means that DHCP Client is waiting Broadcast responses from the DHCP Server. But if it set to “0”, this means that DHCP Client is waiting Unicast responses from the DHCP Sercer. Even if this flag, the DHCP Server acts as its capability. In most cases, the responses of DHCP Server is Unicast.

DHCP Offer Message

All the nodes in the network receive the DHCP Discover Message. But, only the DHCP Server reply to this message with DHCP Offer Message. This DHCP Offer Message is sent as Unicast or Broadcast, according to the Broadcast Flag in DHCP Discover Message and DHCP Server capability.

The meaning of this message is :

“I am a DHCP Server. And I can give you these IP address!”Here, the destination port is UDP 67. This time, the message includes some more information :

• MAC address of DHCP Client,
• IP address of DHCP Server
• MAC address of DHCP Server
• Offered IP address and Subnet Mask

DHCP Request Message

When DHCP Client gets the DHCP Offer Messages, it determines one of the DHCP Server and sends a Broadcast DHCP Request Message that says, it select a DHCP Server. In this DHCP section, DHCP Client can get many offers from the different DHCP Servers. But, it selects only one of them. This is generally the first sender.

This message means that:

“This DHCP Server can be my DHCP Server. Other DHCP Servers! Shut up!”With this DHCP Request Message, DHCP Client says all the nodes that, it has determined the DHCP Server of its dreams. So, others DHCP Servers stop sending DHCP messages. The DHCP Server accepted is also get this DHCP Request Message.

DHCP Ack Message

At this stage, accepted DHCP Server sends a DHCP Ack Message.In this messages assigned IP address and other IP informations are send to the DHCP Client. This message is again can be unicast or broadcast according to the Broadcast Flag in DHCP Discover Message and DHCP Server capability.

This message means that:

“Here is your IP information.”You can find the whole process of a successfull DHCP IP Address Allcoation Operation steps below:

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3 Responses to “DHCP IP Allocation Operation”

Michael Parker / October 12, 2017 / Reply

Hello Gokhan,

This was very informative.

Thanks!

Michael
Ramón Onrubia Pérez / October 15, 2017 / Reply

Hello Gokhan

The explanation has some mistakes. The responses from the server (OFFER and ACK) are not neccesarily broadcast. Most of the time it is unicast.

In the Discover there is a flag called the broadcast flag (in the bootp flags field) that the client uses to tell the server how he would like the offer to be: broadcast if it is on or unicast if it is off but the server makes the final decision based on its capability.

You can see it in a tcpdump o wireshark capture.
- gokhankosem / October 30, 2017 / Reply
  
  Thank you very much for the explanation Ramon ;) I draw an example pictures, but your explanations is better. SO, I ll add your comments to the article. Thanks a lot…