Concept of DHCP

Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to automatically assign IP addresses and other network settings (subnet mask, gateway, DNS) to devices on a network.

It removes the need for manual IP configuration and helps avoid configuration errors.
A DHCP server manages a pool of IPs and provides them to clients using a process called DORA.

DHCP Message Exchange (DORA)

The DHCP IP allocation process consists of four main messages:

1. Discover msg
2. Offer msg
3. Request msg
4. Acknowlegement msg

1. DHCP Discover Message

• The first message sent by the DHCP client.
• Purpose: To locate available DHCP servers.
• Type: Broadcast (255.255.255.255).
• Contains:
  • Client MAC address
  • Request for network configuration
  • Indication that the client has no IP

Explanation:

When a device joins a network, it does not know the DHCP server’s address.
So, it broadcasts a DHCPDISCOVER message to reach all DHCP servers.

2. DHCP Offer Message

• Sent by the DHCP server in response to the discovery message.
• Purpose: To offer an IP address lease to the client.
• Type: Broadcast or unicast.
• Contains:
  • Offered IP address
  • Subnet mask
  • Default gateway
  • DNS servers
  • Lease duration

Explanation:

Any DHCP server receiving the discovery may send a DHCPOFFER.
It informs the client:
“Here is an IP address you can use.”

3. DHCP Request Message

(Sometimes mistakenly called “recovery message,” but the correct term is DHCP Request)

• Sent by the client.
• Purpose: To accept one DHCP offer and reject others.
• Type: Broadcast.
• Contains:
  • Selected DHCP server ID
  • Requested IP address
  • Client MAC address

Explanation:

If multiple offers are received, the client chooses one and broadcasts a DHCPREQUEST, telling all servers:
“I accept the offer from this specific server.”

4. DHCP Acknowledgement (ACK) Message

• Sent by the selected DHCP server.
• Purpose: To confirm the IP lease and finalize configuration.
• Contains:
  • Confirmed IP address
  • Subnet mask
  • Gateway
  • DNS information
  • Lease start time and duration

Explanation:

The DHCPACK message completes the DORA process.
The client configures its IP address and officially joins the network.

Summary Table

Step	Message	Sender	Purpose
1	Discover	Client	Locate DHCP servers
2	Offer	Server	Offer IP address
3	Request	Client	Request chosen IP offer
4	Acknowledge	Server	Confirm IP assignment

DORA = Discover → Offer → Request → Acknowledge