How DHCP Works

We’ve seen that DHCP automatically assigns IP settings to devices.

But how does it actually work behind the scenes?

Let’s break it down together.

DHCP Ports

The DHCP protocol uses specific ports to communicate:

• UDP port 68 — used by the DHCP client

• UDP port 67 — used by the DHCP server

Figure 1 – DHCP communication using UDP port 68 for the client and UDP port 67 for the server.

These ports are used throughout the DHCP process.

Make sure to remember them for your CCNA exam.

The DHCP Process (Step by Step)

When a device connects to a network without an IP address, it uses DHCP to automatically obtain its network configuration.

This happens through a 4-step process often referred to as DORA:

Discover, Offer, Request, Acknowledge

Let’s go through each step.

The first two messages are a search: the client looks for a server, and a server answers.

Step 1 — DHCP Discover

When a device joins a network for the first time, it has no IP address and doesn’t know where the DHCP server is.

Figure 2 – DHCP client wondering where the DHCP server is to request an IP address.

To find one, it sends a DHCP Discover message as a broadcast on the network.

Figure 3 – DHCP client sending a Discover message to locate available DHCP servers.

This way, any DHCP server listening can respond with an offer.
In this capture, we can observe:

Figure 4 – Wireshark capture of a DHCP Discover broadcast using UDP ports 68 and 67.

• Destination MAC: FF:FF:FF:FF:FF:FF (unknown server MAC, so broadcast).

• Source IP: 0.0.0.0 (the client doesn't have an IP address yet).

• Destination IP: 255.255.255.255 (broadcast to the entire subnet).

• UDP Ports: source port 68 (client), destination port 67 (server).

This broadcast is essential because the client has no prior information about the server’s address.

Step 2 — DHCP Offer

When a DHCP server receives the Discover message, it replies with a DHCP Offer, proposing an IP address and configuration settings to the client.

This message is usually sent as a unicast directly to the client’s MAC address.

Figure 5 – DHCP server sending an Offer with an IP address to the client.