DHCP Discovery Process

Overview

This lecture explains the DHCP discovery process, detailing how devices are automatically assigned IP addresses on a network without prior network configuration.

DHCP and the OSI Model

• DHCP (Dynamic Host Configuration Protocol) is an application layer protocol.
• It depends on lower layers (transport, network, data link, physical) to operate.

DHCP Discovery Steps

• The process is called DHCP discovery and includes four main steps: Discover, Offer, Request, and Acknowledgement (DORA).

1. Server Discovery: Client sends a DHCP Discover message as a broadcast because it lacks an IP address and the server’s address.

• DHCP Discover is sent from UDP port 68 to UDP port 67.
• Message is encapsulated in an IP datagram: source 0.0.0.0, destination 255.255.255.255 (broadcast).

2. DHCP Offer: Server replies with a DHCP Offer message, broadcast to the network, specifying the client’s MAC address.

• Sent from port 67 to port 68.
• Source IP: server’s IP; Destination IP: 255.255.255.255.

3. DHCP Request: Client responds with a DHCP Request to accept the offered configuration.

• Still sent with source IP 0.0.0.0 and destination 255.255.255.255.
• Indicates client accepts the offered IP address.

4. DHCP Acknowledgement: Server sends DHCP ACK to assign the IP and configuration.

• Broadcast to 255.255.255.255; includes client’s MAC to identify recipient.

Address Allocation and Lease Management

• Server can allocate addresses dynamically, automatically, or in a fixed manner.
• DHCP lease contains IP configuration and an expiration time.
• When the lease expires, client must repeat the DHCP discovery to renew.
• Client can release its lease when disconnecting, making the IP available again.

Key Terms & Definitions

• DHCP (Dynamic Host Configuration Protocol) — Application protocol to automatically assign IP configuration to network devices.
• Broadcast Address (255.255.255.255) — Special address used to send data to all nodes on a LAN.
• DHCP Lease — Temporary assignment of IP configuration parameters by a DHCP server.
• MAC Address — Unique hardware identifier for network interfaces.

Action Items / Next Steps

• Review the four DHCP discovery steps (Discover, Offer, Request, ACK).
• Understand the format and purpose of DHCP messages.
• Be able to explain the lease renewal and release process.