DORA process in DHCP

DORA Process

1) D - Discover: Client makes a UDP Broadcast to the server with a DHCPDiscover, or Discover packet.

2) O - Offer: DHCP offers to the client. 

The server sends a DHCPOffer including other configuration parameters (DHCP Options) for the client per the servers configuration file

3) R - Request: In response to the offer Client requests the server. 

The client replies DHCPRequest, unicast to the server, requesting the offered address.

4) A - Acknowledgement: The server sends DHCPAck acknowledging the request which is the clients final permission to take the address as offered. Before sending the ack the server double checks that the offered address is still available, that the parameters match the clients request and (if so) marks the address taken. 

DHCP

• Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addresses to devices on a network. 
• With dynamic addressing, a device can have a different IP address every time it connects to the network. 
• In some systems, the device's IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses. 
• Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task. 
• This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. 
• Many ISPs use dynamic IP addressing for dial-up users. 
• The DHCP Server keeps all the information and data base about the DHCP Clients.
• The default port of DHCP is 67, the server listens on port 67 for requests and responses to the client on port 68.

The Concept of Lease

• With all the necessary information on how DHCP works, one should also know that the IP  address assigned by DHCP server to DHCP client is on a lease. 
• After the lease expires the DHCP server is free to assign the same IP address to any other host or device requesting for the same. 
• For example, keeping lease time 8-10 hours is helpful in case of PC’s that are shut down at the end of the day.  
• So, lease has to be renewed from time to time. The DHCP client tries to renew the lease after half of the lease time has expired. 
• This is done by the exchange of DHCPREQUEST and DHCPACK messages. 
• While doing all this, the client enters the renewing stage.