DORA Process Dynamic Host Configuration Protocol
The four-step sequence utilized by the Dynamic Host Configuration Protocol (DHCP) to lease IPv4 addresses, subnet masks, default gateways, and DNS servers to endpoint clients. The process operates over UDP Ports 67 (Server) and 68 (Client).
The 4-Step Negotiation
Discover Broadcast
The client boots up without an IP address. It screams a DHCP Discover message out to the local network (Destination IP: 255.255.255.255, Destination MAC: FF:FF:FF:FF:FF:FF) asking "Are there any DHCP servers out there?"
Offer Unicast
Any DHCP server that hears the broadcast responds directly to the client's burned-in MAC address with a DHCP Offer. This packet contains a proposed IP address from its pool, alongside the lease time, subnet mask, and gateway details.
Request Broadcast
If the client receives multiple offers from different servers, it usually picks the first one it received. Crucially, it broadcasts its Request. It does this to officially claim the accepted IP address, while simultaneously notifying all other rejected DHCP servers that they can put their offered IPs back into their available pools.
Acknowledge Unicast
The winning DHCP server locks the IP address into its binding table and sends a final DHCP ACK to the client. The client applies the IP configuration to its network interface and transitions to a fully operational state.
CCNA Exam Gotchas
The IP Helper Relay (DHCP Relay Agent)
Because the initial Discover message is a Layer 2 broadcast, it will be instantly dropped by the local default gateway (routers do not forward broadcasts). If the DHCP server is on a different subnet than the clients, the DORA process completely fails.
You must overcome this by applying the interface command ip helper-address [server-ip] to the local gateway. This intercepts the Discover broadcast, wraps it inside a unicast packet, and relays it safely across the enterprise to the central server.