What is DHCP? DORA process

What is DHCP?

DHCP stands for Dynamic Host Control Protocol. It is a network management protocol used to dynamically(automatically) assign an IP address to any device which is newly connecting the network.

DHCP assigns IP to the device as well as also configure SubnetMask, Gateway, Primary DNS/ Secondary DNS, etc to the device.

How DHCP works

A DHCP is an application layer protocol it uses UDP port - 67 for the server to use dynamically assign an IP address to the client. Where clients use UDP port - 68 for communication here with IP address subnet mask, gateway, and DNS are also included. 

DHCP is a client-server protocol. which is operated in 4 stages :

1. Server Discovery 
2. IP Lease Offer 
3. IP Lease Request 
4. Acknowledgment.

A user turns on a computer with a DHCP client. The client computer uses a UDP-68 port to send a broadcast request. (Called a DISCOVER).

               The router navigates the DISCOVER packets to the correct DHCP server. The server receives the packet. It uses the UDP-67 port dynamically to assign an IP address if it is available! Then the server sends the packet back to the client with OFFER. With that address information, the client sends the REQUEST packet back, letting the server know that it intends to use the address. The server sends an ACK packet, confirming that the client has been provided this address for a server-specific period of time.

The above fig is DHCP client-server communication process which is also called as DORA process. It stands for Discovery Offer Request and Acknowledgment 

How the DORA process works.

DHCP discovery Message →
This is the first message which is sent by the DHCP client to the DHCP server in the network. whereas, Source IP = DHCP client.
               Destination IP = DHCP server.


DHCP offer process →
In this message, the DHCP server offers an IP address to the DHCP client. This message is unicast at the data link layer but broadcast at the network layer.
whereas, Source IP = DHCP server
               Destination IP = DHCP client.

DHCP request message →
In this message DHCP client request to DHCP server for offering an IP. This message is unicast at the data link layer but broadcast at the network layer.
whereas, Source IP = DHCP client.
               Destination IP = DHCP server.

DHCP Acknowledgement →
This is a final message sent from the DHCP server to the DHCP client that shows that you are now connected to the server.
whereas, Source IP = DHCP server
               Destination IP = DHCP client.

We can configure the DHCP server on several platforms such as Windows Server, CentOS, RHEL, other Linux flavors. It also supports L3 devices: Firewall, Router & L3 Switch.

You may like to read......
1) DHCP server configuration guide Windows & Linux server
2) DHCP server configuration guide for L3 device and Firewall.