Dynamic Host Configuration Protocol (DHCP) Server

1. Overview

A DHCP Server is a network server that automatically provides and assigns IP addresses, default gateways and other network parameters to client devices. It relies on the standard protocol known as Dynamic Host Configuration Protocol or DHCP to respond to broadcast queries by clients.

A DHCP server automatically sends the required network parameters for clients to properly communicate on the network. Without it, the network administrator has to manually set up every client that joins the network, which can be cumbersome, especially in large networks. DHCP servers usually assign each client with a unique dynamic IP address, which changes when the client’s lease for that IP address has expired.

2. Terminology

2.1 DHCP Address Pool

DHCP address pool is a virtual container that contains all the IP addresses that have been configured in the DHCP range to make available to the client computers. As soon as an IP address from the address pool is assigned to a client computer, the address is temporarily removed from the pool.

2.2 DHCP Lease

When the DHCP server assigns an IP address to a DHCP client computer, the address is assigned for a specific time duration. The time duration for which an IP address is assigned to a DHCP client computer by the DHCP server is technically called the DHCP lease. When the DHCP lease expires, the IP address is revoked from the DHCP client computer and is sent back to the DHCP address pool.

3. DORA

The step-by-step process through which a DHCP server assigns an IP address to a DHCP client computer is in short called DORA. Once the entire DORA process is complete, the DHCP server marks the IP address as assigned in its database, and the DHCP client computer starts using the assigned IP address to communicate with other computers in the network.

3.1 D

D in the term DORA stands for the DHCP Discover packet. The DHCP Discover packet
is broadcasted by the DHCP client computer in order to find the available DHCP server(s) in the network. Since the DHCP client computer sends the DHCP Discover packet as a broadcast, all the
DHCP servers that are present in the network receive the packet and respond accordingly.

3.2 O

O in the term DORA stands for the DHCP Offer packet. The DHCP Offer is a unicast packet that is sent by the DHCP server who receives the DHCP Discover packet from the DHCP client computer. The DHCP Offer packet contains the available IP address that the DHCP server offers to the client computer.

3.3 R

R in the term DORA stands for the DHCP Request packet. The DHCP Request is again a unicast packet sent by the DHCP client computer to the DHCP server who had sent the DHCP Offer packet to the client computer. With the help of the DHCP Request packet, the DHCP client computer requests the DHCP server to assign the offered IP address to it.

3.4 A

A in the term DORA stands for the DHCP Acknowledge packet. The DHCP Acknowledge is a unicast packet sent by the DHCP server to the DHCP client computer acknowledging the assignment of the offered IP address for a specific lease time.

4. Reference

https://www.infoblox.com/glossary/dhcp-server/

https://forums.tomshardware.com/faq/how-dhcp-server-assigns-ip-addresses-to-the-client-computers.1612744/