# DHCP - Dynamic Host Configuration Protocol
# IP Address assignment
- MAC Address (6 bytes) : data link layer (L3)
- first 3 bytes: Organizational Unique Identifier
- last 3 bytes: Vendor Assigned (NIC Cards, Interfaces)
- IP Network (4 bytes): network layer (L2)
# Dynamic addressing - DHCP
DHCP is a network protocol that automatically assigns IP addresses and other network settings to devices when they join a network.
Without DHCP, every device would need manual IP configuration, which is error-prone and inefficient.
# What DHCP provides
When a device connects to a network, DHCP can assign:
| Setting | Purpose |
|---|---|
| IP address | Unique network identifier for the device |
| Subnet mask | Defines the local network range |
| Default gateway | IP of router to reach other networks |
| DNS server | IP addresses of DNS servers for name resolution |
| Lease time | Duration the IP assignment is valid |
| Optional: NTP server, domain name, WINS | Additional network configs |
# How it works
- DHCP Discover (Client → Broadcast)
- Device asks: “Is there a DHCP server on the network?”
- DHCP Offer (Server → Broadcast/Unicast)
- Server replies with: “Here is an available IP and settings.”
- DHCP Request (Client → Server)
- Client requests the offered IP: “I want this IP lease, please.”
- DHCP Acknowledge (ACK) (Server → Client)
- Server confirms lease: “You can use this IP for X seconds/minutes.”
This is commonly called DORA (Discover → Offer → Request → Acknowledge).
Example Scenario:
Laptop connects to Wi-Fi.
- Laptop broadcasts DHCP Discover.
- Router/AP acting as DHCP server replies:
“You can use 192.168.1.20, subnet 255.255.255.0, gateway 192.168.1.1, DNS 8.8.8.8, lease 24h.” - Laptop sends DHCP Request for 192.168.1.20.
- Router replies ACK → laptop can now communicate on the network.
# Types of IP Assignment
| Method | Description |
|---|---|
| Dynamic | IP assigned temporarily by DHCP server (typical for most clients). |
| Automatic | IP assigned permanently by DHCP server from pool (like static lease). |
| Manual / Static | Admin manually binds IP to a device’s MAC, but DHCP still manages it. |
# DHCP Layers
| Layer (OSI) | How DHCP Uses It |
|---|---|
| Application (L7) | DHCP itself is an application protocol — defines the DORA process and data structures (options, IP assignment). |
| Transport (L4) | Uses UDP: - Client → Server: port 68 - Server → Client: port 67 |
| Network (L3) | DHCP messages are encapsulated in IP packets. - Initial client broadcast has source IP 0.0.0.0 (client doesn’t have IP yet) |
| Data Link (L2) | DHCP messages are delivered over Ethernet/Wi-Fi frames. Broadcast MAC ff:ff:ff:ff:ff:ff for Discover / Offer messages |
Key Notes
- Primary layer: Application (L7) — DHCP is a protocol that runs over UDP/IP.
- Needs L2/L3 support because a client doesn’t have an IP initially — broadcast is used to reach DHCP server.
- DHCP relies on ARP later to map assigned IPs to MAC addresses for normal communication.