DHCP is one of the most essential services in any TCP/IP network. Instead of manually assigning IP addresses to every device, DHCP automatically handles the network configuration.
Zero-Touch Config
DHCP reduces human error, simplifies administration, and allows devices to connect instantly when they join a network segment.
Why DHCP?
- Centralized Management: Scope and lease settings are controlled in one place.
- Dynamic Allocation: Addresses are leased, not permanently occupied.
- Mobility: Devices can move between subnets and automatically get correct settings.
The DORA Process
DHCP communication follows a four-step handshake known as DORA:
| Step | Message | Action |
|---|---|---|
| 1 | Discover | Client broadcasts to find a DHCP server. |
| 2 | Offer | Server offers an available IP address to the client. |
| 3 | Request | Client requests the offered IP from the server. |
| 4 | Acknowledgment | Server confirms and reserves the IP for the client's MAC. |
DHCP Relay
Since DHCP Discover is a broadcast, it doesn't cross routers. To have one DHCP server support multiple subnets, you must configure a DHCP Relay (ip helper-address) on the router interface.
Configuration Example
Router(config)# ip dhcp pool MY_POOL Router(dhcp-config)# network 192.168.1.0 255.255.255.0 Router(dhcp-config)# default-router 192.168.1.1 Router(dhcp-config)# dns-server 8.8.8.8