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What is a DHCP Server?

Article :: KB00026

A DHCP (Dynamic Host Configuration Protocol) Server is a network service that automatically assigns IP addresses and other network configuration settings—like subnet mask, default gateway, and DNS servers—to devices (clients) on a network.


Instead of manually configuring each device, the DHCP server streamlines the process by:


  • 📡 Listening for IP requests from devices joining the network

  • 🧾 Offering available IP addresses from a defined pool (called a scope)

  • ⏳ Leasing the IP address to the device for a specific duration

  • 🔁 Reclaiming and reassigning IPs when leases expire or devices disconnect


This automation is especially useful in enterprise environments, where managing hundreds or thousands of devices manually would be impractical.


DHCP Process Diagram
DHCP Process Diagram


🧾 DORA Process Explained

In DHCP, this is the four-step conversation between a client and the DHCP server when assigning an IP address. Here's how it unfolds:


1. Discover 📡

The client device broadcasts a DHCP Discover message on the network to say:


"Hi! Is there any DHCP server out there? I need an IP address!"


  • This is a broadcast message because the client doesn’t know any server's IP yet.

  • Sent on UDP port 67 by the client, destined for port 68 on the server.


2. Offer 🎁

DHCP servers that receive the Discover message respond with a DHCP Offer:


"Hello! I'm a DHCP server. I can offer you this IP address (e.g., 192.168.10.101) along with other network settings."


  • The offer includes:

    • Proposed IP address

    • Subnet mask

    • Lease duration

    • Gateway (default router)

    • DNS servers

  • If multiple DHCP servers exist, the client may receive multiple offers.


3. Request 🙋‍♀️

The client chooses one of the offers and sends a DHCP Request:


"Thanks, I’ll take that offer from Server X!"


  • The request also notifies other DHCP servers that their offers were declined.

  • This is again a broadcast so all servers know which one was chosen.


4. Acknowledge 📨

The selected DHCP server confirms the lease by sending a DHCP Acknowledgement (ACK):


"All set! That IP is now yours to use for the next [X] hours."


  • The client then applies the IP address and related settings.

  • The lease timer starts, and the device is officially connected to the network.


🔄 Lease Renewal

  • Halfway through the lease duration, the client tries to renew it by sending a unicast DHCP Request directly to the same DHCP server.

  • If the server responds with another ACK, the lease is extended without going through DORA again.



The DHCP DORA process uses specific UDP ports for communication between the client and the server. Here's the breakdown:


🔌 DHCP Ports Used in the DORA Process

Message

Direction

Source Port

Destination Port

Protocol

Discover

Client → Broadcast

UDP 68

UDP 67

UDP

Offer

Server → Client (broadcast/unicast)

UDP 67

UDP 68

UDP

Request

Client → Broadcast

UDP 68

UDP 67

UDP

Acknowledge

Server → Client (broadcast/unicast)

UDP 67

UDP 68

UDP

🧠 Quick Notes:

  • UDP Port 67 is used by the DHCP server to listen for client requests.

  • UDP Port 68 is used by the DHCP client to receive responses from the server.

  • All messages are sent over UDP, not TCP, because DHCP is a connectionless protocol.

  • The initial messages are broadcasts because the client doesn’t yet have an IP address.




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