What you’ll learn
- Describe the four-message DORA exchange
- Explain lease renewal at T1 and rebinding at T2
- Recognize the role of a DHCP relay
Why DHCP starts with broadcast
A new client does not yet know its IP address, subnet, or DHCP server. It begins by broadcasting a DHCPDISCOVER message on the local network. One or more servers can respond with a DHCPOFFER containing a proposed address and configuration options.
The client broadcasts a DHCPREQUEST identifying the offer it selected. The chosen server commits the lease and replies with DHCPACK. This Discover, Offer, Request, Acknowledge sequence is commonly remembered as DORA.
The lease continues after DORA
A lease is temporary. At the T1 timer, normally halfway through the lease, the client tries to renew directly with the server that granted it. If that fails, the client reaches T2 and broadcasts a rebinding request so another available server can respond. If the lease expires without renewal, the client must stop using the address.
- T1: renew with the original server
- T2: rebind with any responding server
- Expiry: stop using the leased address and start again
Crossing subnet boundaries
Routers normally do not forward broadcasts, so enterprise networks use DHCP relays. A relay receives the client broadcast, adds information about the client’s network, and forwards the request to a DHCP server. The server uses that context to choose the correct scope or subnet.
Relay configuration is one of the first places to look when an entire subnet cannot obtain leases. If only one client fails, inspect client state, reservations, pool availability, policy, and conflict detection.
What DDI adds
When DHCP and IPAM share state, operators can see pool utilization, reservations, active leases, and address history together. When DHCP also performs controlled dynamic DNS updates, a device’s name and address lifecycle remain aligned.
A successful ping does not prove DHCP is healthy; verify the lease, options, timers, and server identity.
References and further reading
KnowDDI lessons are explanatory guides. Standards and primary documentation remain the authority for implementation decisions.