alt text

DHCP Discovery (Client Side) – Layer by Layer

  1. Application Layer

    • Client says: “I need an IP”
    • DHCP software creates the request.

  2. Transport Layer (UDP)

    • Source Port (68): Reserved port for DHCP clients.
    • Destination Port (67): DHCP Server port(the router)

  3. Network Layer (IP)

    • Source IP: 0.0.0.0 (because the client has no IP yet)
    • Destination IP: 255.255.255.255 (broadcast ip, because client doesn’t know server/router IP)

  4. Data Link Layer (MAC)

    • Source MAC: Client NIC MAC (e.g., A:B:C:D:E:F)
    • Destination MAC: Broadcast (FF:FF:FF:FF:FF:FF)

  5. Physical Layer & NIC

    • NIC converts frame to electrical/wireless signals and sent on network.

Step-by-Step Flow of DHCP Discovery

Layer Information Checked Device Action (Non-Router) Router Action
Data Link MAC: FF:FF:FF:FF:FF:FF "It's a broadcast, I'll take it." "It's a broadcast, I'll take it."
Network IP: 255.255.255.255 "It's a broadcast, keep going." "It's a broadcast, keep going."
Transport Port: 67 "I don't have this port open. DROP." "I am a DHCP Server. ACCEPT."

Key Clarification:

  • Broadcast goes everywhere, but Transport Layer filtering (port numbers) ensures only the DHCP server processes it.

  • Other devices just drop the packet because nothing listens on UDP 67.