How DHCP Works: The DORA Process Explained
DHCP (Dynamic Host Configuration Protocol) automatically gives devices their IP address, subnet mask, default gateway and DNS server. It works through the four-step DORA exchange: Discover, Offer, Request, Acknowledge.
The DORA process
- Discover — the client broadcasts "is there a DHCP server?"
- Offer — a server replies with an available IP and settings.
- Request — the client broadcasts acceptance of that offer.
- Acknowledge — the server confirms and records the lease.
DORA is a classic interview question — memorise the order.
DHCP across subnets: the relay agent
DHCP relies on broadcasts, which routers don't forward. To let clients get addresses from a central server on another subnet, you configure a DHCP relay (ip helper-address) on the router interface — it forwards the requests as unicast to the server. This is a common real-world and exam scenario.
Frequently asked questions
What does DORA stand for in DHCP?
Discover, Offer, Request, Acknowledge — the four messages exchanged when a client obtains an IP lease from a DHCP server.
What information does DHCP provide?
Typically an IP address, subnet mask, default gateway and DNS server addresses — and optionally items like NTP or TFTP servers.
How does DHCP work across different subnets?
With a DHCP relay agent (ip helper-address) on the router, which forwards the client's broadcast requests as unicast to a central DHCP server.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.