Describe the steps taken by the client and DHCP server in order to obtain an IP address

DHCP job is to assign IP addresses , subnet mask , gateway and other options to clients requesting DHCP service.
The process of getting an IP address from a DHCP server involves 4 steps and called DORA.
DHCP uses port UDP 67 DORA stands for Discover Offer Reqest Ack First the client sends DHCPDISCOVER broadcast (IP 255.255.255.255) message.
If there is one or more DHCP servers on the network , they will hear the request on port 67.
Second the existing DHCP server/s on the network reply with an IP lease -DHCPOFFER-, the reply uses the broadcast address 255.255.255.255 but the frame contains the MAC address of the sending Client , so only the requesting device receives the offer.
There may be more than one DHCP server , so the requesting client may receive more than one DHCPOFFER , and usually will use the first one, then it sends an DHCPREQUEST to the DHCP server that sends the offer.
finally , the DHCP server sends back an DHCPACK to inform the client that it can use the offered information.

When a DHCP-configured client connects to a network, the DHCP client sends a broadcast query requesting necessary information of the network to a DHCP server.
The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the name servers, other servers such as time servers, and so forth.
On receiving a valid request, the server assigns the computer an IP address, a lease and other IP configuration parameters, such as the subnet mask and the default gateway.
The query is typically initiated immediately after booting, and must complete before the client can initiate IP-based communication with other hosts.

This process takes four steps,

 

1. DHCPDISCOVER : From DHCP Client to DHCP Server, to find the DHCP server on the local network

2. DHCPOFFER : From DHCP Server to DHCP Client, to offer the IP Parameters

3. DHCPREQUEST: From DHCP Client to DHCP Server, giving a request to get the offered IP Parameters

4. DHCPACK: From DHCP server to DHCP Client, giving confirmation to use the the offered IP parameters.

 

IP parameters includes IP address, subnet mask, DNS address, Default gateway, WINS address etc

 

 

The following way a client obtain IP address from the DHCP server

• When a DHCP client first boots up, it broadcasts a DHCPDiscover message, searching for a DHCP server.
• If  a DHCP server exists on the local segment, it will respond with a DHCPOffer, containing the "offered" IP address, subnet mask etc.
• Once the client receives the offer, it will respond with a DHCPRequest, indicating that it will accept the offered protocol information.
• Finally, the server responds with DHCPAck, acknowledging the client acceptance of offered protocol information.

DORA

Discovery, offer, Request , Acknowledge

 

• DHCP is (generally) a broadcast protocol, so the client doesn't need to know anything about the network, it simply broadcasts a request.

  DHCP servers on the network then see that request and respond to it. the process accur to get an ip by client is DORA

• D iscovery
• O ffer
• R equest
• A cknowledgement

As for the gateway, it can come into play only if it explicitly forwards DHCP traffic to another subnet. That feature is usually called DHCP Relay. Otherwise the gateway ignores the broadcast DHCP message just like it ignores every other broadcast message.

Firstly, a DHCP server is needed which distributes the IP addresses.
This machine will serve as a base for all DHCP requests and must also have a fixed IP address.
Therefore, on a network there can only be one machine with a fixed IP address, the DHCP server.
The basic communication system is BOOTP (with UDP frame).
When a machine is turned on, it has no information on its network configuration, and there is nothing special that the user needs to do to find an IP address.
In order to do this, the technique used is the broadcast: to find and talk with a DHCP server, the machine will simply send a special broadcast packet (broadcast on 255.255.255.255 with other information such as the type of request, the connection ports, etc.) over the local network.
When the DHCP receives the broadcast packet, it will send back another broadcast packet (do not forget that the client does not obviously have the IP address and that therefore cannot be connected to directly) containing all the information required for the client.
It could be believed that one single packet is enough for the protocol to work.
In fact, there are several types of DHCP packets likely to be issued either by the client for the server(s), or by the servers to a client: 1-DHCPDISCOVER (to locate available DHCP servers) 2-DHCPOFFER (server response to a DHCPDISCOVER packet, which contains the initial parameters) 3-DHCPREQUEST (varied request from the client for example to extend its lease) 4-DHCPACK (server response which contains the parameters and IP address of the client) 5-DHCPNAK (server response to indicate to the client that its lease has expired or if the client announces a bad network configuration) DHCPDECLINE (the client announces to the server that the address is already in use) DHCPRELEASE (the client releases its IP address) DHCPINFORM (the client requests local parameters, it already has its IP address) The first packet issued by the client is a DHCPDISCOVER type packet.
The server responds with a DHCPOFFER packet, particularly in order to submit an IP address to the client.
The client establishes its configuration and then makes a DHCPREQUEST to validate its IP address (a broadcast request since DHCPOFFER does not contain the IP address).
The server simply responds with a DHCPACK with the IP address for confirmation of the allocation.
Normally, this is enough for the client to obtain an effective network configuration, but it can be longer or shorter according to whether the client accepts the IP address or not.

DHCP Server is used DORA Rules Discover offer request acknowledgement.
with the help of DORA Rules  it provides ip address to the client.
And one of the most important of DHCP port No.67  and Client Port No.68 and DHCP is used UDP Protocol.
Simply I can say DHCP is used provide IP address to the Client with the Help of DORA Rules as well MAC (Media Access Control) 

DORA This process of assigning the IP addresses by the DHCP server also known as DORA (Discover Offer Request and Acknowledgement).
Client makes a UDP Broadcast to the server about the DHCP discovery.
DHCP offers to the client.
In response to the offer Client requests the server.
Server responds all the Ip Add/mask/gty/dns/wins info along with the acknowledgement packet.
At least one DHCP server must exist on a network.
Once the DHCP server software is installed, you create a DHCP scope, which is a pool of IP addresses that the server manages.
When clients log on, they request an IP address from the server, and the server provides an IP address from its pool of available addresses.
DHCP was originally defined in RFC 1531 (Dynamic Host Configuration Protocol, October 1993) but the most recent update is RFC 2131 (Dynamic Host Configuration Protocol, March 1997).
The IETF Dynamic Host Configuration (dhc) Working Group is chartered to produce a protocol for automated allocation, configuration, and management of IP addresses and TCP/IP protocol stack parameters.

This is DORA Process D = Discover O = Offer R = Request A = Ack Discover = Clent Locates a DHCP Server Offer = DHCP Server offers an IP address Request = Client ask for Address Ack = Server Acknowledges and issues lease DHCP Renewal Process Request Ack

DHCP Client uses a DORA Method i.e.,Discovering the server,offers a particular IP address,Requesting and finally Acknowledgement.This is how server issues IP address to th client.