How DHCP Works: The DORA Process
DHCP hands out IP addresses automatically through four messages known as DORA: the client broadcasts a Discover, a server sends an Offer, the client sends a Request for that offer, and the server sends an Acknowledge. After DORA the device has a working IP, mask, gateway and DNS.
DORA, message by message
D — Discover Client broadcasts "any DHCP server out there?" (src 0.0.0.0 → 255.255.255.255) O — Offer Server replies with an available IP + lease terms R — Request Client broadcasts "I'll take that one" (locks it, tells others) A — Acknowledge Server confirms; client applies IP, mask, gateway, DNS
Discover and Request are broadcasts because the client has no IP yet and doesn't know which offer it will accept — broadcasting lets every server hear the decision, so declined offers are released.
Why the Request is a broadcast, not a reply
A subtle exam favourite: after receiving Offers, the client broadcasts its Request rather than unicasting it. That's deliberate — on a network with multiple DHCP servers, the broadcast tells the losing servers their offer was declined so they can return that address to the pool.
Lease, renewal and expiry
Addresses are leased, not permanent. At 50% of the lease (T1) the client unicasts a renewal to its server; if that fails, at 87.5% (T2) it broadcasts to any server. Let a lease lapse entirely and the client must start DORA again. This is why a device offline for a while sometimes gets a different IP back.
DHCP across subnets: the relay
DHCP relies on broadcasts, but routers don't forward broadcasts — so how does a client get an address from a server on another subnet? A DHCP relay agent (Cisco ip helper-address) on the router catches the Discover and unicasts it to the server, then relays the reply back. Without it, every subnet would need its own server. Configure it in the DHCP config guide.
When DHCP fails
No DHCP server reachable → the device self-assigns a 169.254.x.x APIPA address and has no real connectivity. Two servers with overlapping pools → an IP conflict. Rogue servers on the LAN are blocked with DHCP snooping.
Frequently asked questions
What does DORA stand for in DHCP?
Discover, Offer, Request, Acknowledge — the four messages exchanged when a client obtains an IP address from a DHCP server.
Why are DHCP Discover and Request messages broadcasts?
The client has no IP address yet and does not know which server it will choose, so it broadcasts. The Request broadcast also tells non-selected servers to release the address they offered.
What is a DHCP lease?
A time-limited assignment of an IP address. The client renews at 50% of the lease and, if that fails, tries any server at 87.5%. If the lease expires, the client restarts the DORA process.
How does DHCP work across different subnets?
A DHCP relay agent (ip helper-address on the router) forwards the client's broadcast to a DHCP server on another subnet as a unicast, and relays the reply back — so one server can serve many subnets.
What does a 169.254 address mean?
It is an APIPA self-assigned address, used when no DHCP server can be reached. It signals that DHCP failed and the device has no normal network connectivity.
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