What Is DHCP?
DHCP (Dynamic Host Configuration Protocol) is a network protocol that automatically assigns IP addresses and other network configuration parameters to devices when they connect to a network. Without DHCP, every device on a network would need to be manually configured with a unique IP address, subnet mask, gateway, and DNS server - A significant administrative burden for any network larger than a handful of devices.
The DHCP Lease Process (DORA)
| Step | Name | Who Sends | What Happens |
|---|---|---|---|
| 1 | Discover | Client (new device) | Broadcasts a DHCPDISCOVER message to find available DHCP servers on the network |
| 2 | Offer | DHCP Server (router) | Responds with a DHCPOFFER containing a proposed IP address, subnet mask, lease time, gateway, and DNS |
| 3 | Request | Client | Broadcasts a DHCPREQUEST accepting the offer (also informs other DHCP servers their offer was declined) |
| 4 | Acknowledge | DHCP Server | Sends a DHCPACK confirming the lease - The client can now use the assigned IP for the lease duration |
DHCP Lease Times
Every DHCP assignment has a lease time - The period for which the IP assignment is valid. Before the lease expires, the client sends a renewal request (DHCPREQUEST) to keep the same IP. Typical lease times: home routers default to 24 hours; corporate networks often use 8 hours; mobile hotspots may use as little as 1 hour. Short lease times mean IPs are recycled faster; long lease times reduce DHCP traffic.
Static vs Dynamic IP Assignment
- Dynamic assignment: the DHCP server assigns an available IP from its pool - The device may get a different IP on the next connection.
- Static IP (manual): configured on the device itself, bypassing DHCP entirely - Must be chosen outside the router's DHCP pool to avoid conflicts.
- DHCP reservation (static DHCP): configured in the router to always assign the same IP to a specific MAC address - The device still uses DHCP but always gets the same address. Best practice for devices requiring a fixed IP (printers, servers, consoles for port forwarding).
- DHCP scope: the range of IPs the server is configured to distribute, e.g. 192.168.1.100 to 192.168.1.200.
- DHCP conflict: if a device is manually configured with an IP already in use, both devices will have communication issues - Always check the DHCP pool before setting static IPs.