PlantUML DiagramDHCP DORA
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A user connects their laptop to the office network and gets an IP in seconds — no configuration, no prompts. Behind that seamless experience, four network messages travel between the laptop and a DHCP server that might be on a completely different subnet. Understanding those four messages is the foundation of troubleshooting every DHCP issue you'll ever see.
DHCP replaced the manual bookkeeping of IP assignments — one spreadsheet per subnet, updating it every time someone moved. The protocol automates the assignment, tracks leases, and handles reclamation when devices leave. It's the reason you can plug a laptop into any port and be online immediately.
Four messages: DISCOVER, OFFER, REQUEST, ACK — DORA. The client broadcasts DISCOVER because it has no IP and doesn't know where servers are. The server responds with an OFFER containing a proposed IP and lease options. The client broadcasts REQUEST to confirm (broadcast so all servers hear it and can withdraw their offers). The server sends ACK to finalize.
Notice the client broadcasts REQUEST even though it has just been offered an IP. Why? Because multiple servers may have offered. Broadcasting the REQUEST tells all servers which offer was accepted, so the others can withdraw theirs. It's the polite way to say 'I chose this one, thanks anyway.'
| Phase | When it happens | Message |
|---|---|---|
| Initial | Client boots, needs address | DORA |
| Renewal (T1) | 50% of lease has elapsed | Unicast REQUEST to original server |
| Rebinding (T2) | 87.5% of lease has elapsed | Broadcast REQUEST to any server |
| Expiration | 100% of lease has elapsed | Client gives up, restarts DORA |
The renewal thresholds are standardized: T1 at 50%, T2 at 87.5%. Between T1 and T2, the client tries the original server. Between T2 and expiry, it broadcasts in case a different server can renew. Past expiry, the client releases its IP and starts over — usually causing a brief outage.
DHCP automates address assignment via DORA. Clients get IP, mask, gateway, DNS, and other options. Leases renew at 50% and rebind at 87.5%. Next lesson covers configuring DHCP on Cisco IOS.
Finish the lesson once you have worked through the material. This awards ★ 15 XP.