How DORA Works? - GeeksforGeeks

16 Dec, 2021
Dynamic Host Configuration Protocol(DHCP) uses the DORA. Dynamic Host Configuration Protocol is the protocol of the application layer. It is used to provide Subnet Mask, Router Address, DNS Address, and Vendor Class Identifier. In fact, DHCP provides an automatic IP address to Hosts which want to connect to a network.
DORA is the process that is used by DHCP. DORA helps in providing an IP address to hosts or client machines. DORA is the process that follows some steps between the server and client. It gets the IP address from the centralized server. It consists of four-stage:
Discover
Offer
Request
Acknowledge
Now let’s see what happens when DHCP clients request a DHCP server an IP address. Let’s see what messages are exchanged between them in the process.
Note: In the network layer, DHCP messages are always broadcast. In the data link layer also DHCP messages are broadcast.
This is the first message in the DORA process which helps in finding the DHCP server of the network. DHCP client will find the server by sending DHCP discover message. The broadcast message is sent to the network. As the DHCP client doesn’t know the IP address of the server so the message is broadcast with a destination IP is 255.255.255.255. And the source IP will be 0.0.0.0 as the client does not have any IP address. Here the DHCP discover message in the data link layer and network layer is always broadcast. 
Source IP address: 0.0.0.0  
Destination IP address: 255.255.255.255
Source MAC address: MAC address of DHCP clients
Destination MAC address: FF:FF:FF:FF:FF:FF
DHCP server receives the discover message and it replays the DHCP client with the DHCP offer request. The server sends a DHCP offer message with filled information. It has information about the IP address and duration of time that a host can use. Here destination IP address will be 255.255.255.255 as the DHCP client still does not have its IP address. But this DHCP offer message is broadcast in the network layer and unicast in the data link layer. 
Source IP address: IP Address of DHCP Server
Destination IP address: 255.255.255.255
Source MAC address: MAC address of DHCP Server
Destination MAC address: MAC address of DHCP clients
DHCP clients send the request message to the server when it receives a DHCP offer message from the server. This message tells the server that it accepts the IP address given by the server. Here destination address will be 255.255.255.255 means it’s again broadcast. The reason for this is there might be many DHCP servers in the network so the client may receive multiple offer messages and it will accept the request that reaches him first and send a broadcast message to eliminate other DHCP servers. Here source IP address will be 0.0.0.0 as the DHCP server hasn’t yet assigned an IP address to the client. DHCP Request Message is also a broadcast message.
Source IP address: 0.0.0.0
Destination IP address: 255.255.255.255
Source MAC address: MAC address of DHCP clients
Destination MAC address: MAC address of DHCP server
This is the last step or message in the DORA process. The DHCP server sends Acknowledge Message to the client when it receives the request message from the DHCP client. This message will contain the IP address and subnet mask that the server assigns to the client. Source IP address will be the IP address of the server. This will be again broadcast message as the destination IP address is 255.255.255.255. But it is unicast in the case of the data link layer. 
Source IP address: IP Address of DHCP Server
Destination IP address: 255.255.255.255
Source MAC address: MAC address of DHCP server
Destination MAC address: MAC address of DHCP clients
So, this is the DORA process and when this process is over DHCP client will get its IP address. Here things to remember is
DHCP Discover Message – Broadcast
DHCP Offer Message – Broadcast in the network layer and unicast in the data link layer
DHCP Request Message – Broadcast in the network layer and unicast in the data link layer
DHCP Acknowledge Message – Broadcast in the network layer and unicast in the data link layer
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