Basics: DHCP Client IP address Process

Today we are going to talk about the process by which DHCP client get the IP address from the DHCP server. The process is very important to understand and is widely asked in the interview.

Let's talk about the Process now. the whole process will be explained in four steps as described below to understand easily. 

Fig 1.1- DHCP Client/Server

Lease Request:
The first step is the Lease request where the client initializes a limited version of TCP/IP and broadcasts a DHCPDISCOVER packet requesting the location of a DHCP server.

Lease Offer:
The second step is Lease Offer where all DHCP servers with available IP addresses send DHCPOFFER packets to the client. These include the client's hardware address, the IP address the server is offering, the subnet mask, the duration of the IP lease, and the IP address of the DHCP server making the offer.

Lease Selection:
The third step is the lease selection where the client selects the IP address from the first offer it receives and broadcasts a DHCPREQUEST packet requesting to lease the IP address in that offer.

IP Lease Acknowledgment:
The last step is IP Lease Acknowledgment, where the DHCP server that made the offer responds and all other DHCP servers withdraw their offers. The IP addressing information is assigned to the client and the offering DHCP server sends a DHCPACK (acknowledgement) packet directly to the client. The client finishes initializing and binding the TCP/IP protocol.

Important Fact

Remember that DHCP lease process is a frame-level broadcasting and DHCP requests typically do not pass through routers to other subnets.

So now, if you want to use the DHCP request beyond the subnet, you need to enable BootP ( which is a DHCP broadcast) and Configure your laptop/PC for BootP forwarding to request IP information on behalf of other clients.