What is a DHCP lease?
A DHCP lease is a binding between a client and network configuration supplied by a DHCP server. For IPv4, that configuration commonly includes an address, subnet mask, default gateway, DNS servers, and a lease value. Dynamic leases normally have finite lifetimes, but RFC 2131 also permits an infinite lease and defines automatic allocation as permanent. The word “lease” records how the server assigns that local configuration; it does not transfer ownership or registry control of an IP address.
If “IP lease” means a commercial agreement to use public IPv4 address space, read the IPv4 leasing guide. A commercial IPv4 lease needs verified resource authority, routing, contract, reputation, renewal, and return terms. A DHCP lease is a network-configuration record and solves a different problem.
| Question | DHCP lease | Commercial IPv4 lease |
|---|---|---|
| What is provided? | Address and host configuration inside a network scope | Contractual use of an exact public IPv4 prefix |
| Who controls it? | The local network's DHCP administration | The resource holder or an authorized lessor, with separate routing parties |
| How long does it last? | A finite lease can renew or expire; an RFC infinite lease has no expiration timer. Release or administrative change can still end use. | For the term and renewal conditions in the contract |
| What does it prove? | Current configuration state recorded by that DHCP system | Only the rights stated in the verified agreement; registry and routing evidence remain separate |
How does DHCP allocate an IP address?
RFC 2131 defines DHCP as a client-server protocol that delivers host configuration and supports address allocation. When an IPv4 client does not already have a usable binding, the common exchange is summarized as Discover, Offer, Request, and Acknowledge—often called DORA.
| Step | Message | What happens | What to record when troubleshooting |
|---|---|---|---|
| 1 | DHCPDISCOVER | The client looks for available DHCP servers and configuration. | Client identifier, interface, VLAN or subnet, transaction ID, and relay path |
| 2 | DHCPOFFER | A server proposes an address and configuration from the policy for that client and scope. | Offering server, proposed address, subnet, lease time, and options |
| 3 | DHCPREQUEST | The client identifies the selected offer or requests continued use of a prior address. | Selected server, requested address, client state, and whether the request was broadcast or unicast |
| 4 | DHCPACK | The server confirms the binding and supplies the configuration the client may apply. | Lease start, lifetime, T1, T2, gateway, DNS, and server identifier |
DORA is a useful summary, not the entire protocol. A server can send DHCPNAK when the requested configuration is invalid for the current network. A client can send DHCPDECLINE after detecting that an offered IPv4 address is already in use, or DHCPRELEASE when it gives up a binding. Reboot, reconnect, renewal, rebinding, and information-only requests use related paths.
Dynamic, automatic, and manual DHCP allocation
RFC 2131 gives “automatic,” “dynamic,” and “manual” specific meanings. They are easy to blur in product interfaces, so check what the actual server does rather than relying on one label.
| Mechanism | Selection and lifetime | Typical use | Important boundary |
|---|---|---|---|
| Dynamic allocation | The server assigns an address for a limited period or until release, then can reuse it. | Client devices and workloads that join, leave, or move | The client may receive the same address again, but should not assume permanence. |
| Automatic allocation | The server assigns a permanent address to a client under the RFC's terminology. | Managed environments that want server-controlled persistent bindings | “Automatic” does not mean a short dynamic lease in the standard's terminology. |
| Manual allocation or reservation | An administrator selects the address and DHCP conveys it to the identified client. | Infrastructure that needs predictable local addressing | The identifier, exclusion, ownership, and change record must stay current. |
A manually configured static address sits outside the DHCP allocation path. Keep it outside the dynamic pool or explicitly excluded, document its prefix and gateway, and check for overlap. A DHCP reservation provides predictable configuration while retaining server-side records; it is not the same as typing a static address into the host.
What happens during DHCP lease renewal?
For a finite DHCPv4 lease, the common renewal timeline is:
- Bound: after a valid acknowledgement and any required local checks, the client uses the configuration for the lease lifetime.
- Renewing at T1: the client normally asks the server that granted the lease to extend it. The server can configure T1; when it does not, RFC 2131 defines a default of 50% of the lease duration.
- Rebinding at T2: if renewal does not succeed, the client broadcasts a request so an available server can extend the lease. The configurable T2 default is 87.5% of the lease duration.
- Expiration: without a successful extension, the client must stop using the expired address and return to address acquisition. Applications can lose connectivity even when the physical link remains up.
An infinite DHCPv4 lease has no expiration deadline and does not require T1/T2 renewal solely to extend that lifetime. It still does not create address ownership, and client or server configuration can still change.
For finite leases, T1 and T2 are protocol timers, not service-level guarantees. Server failover, relay reachability, clock behavior, client sleep, network changes, and implementation policy affect what operators observe. Troubleshoot from packet and server records with timestamps instead of assuming that “half the lease” means the client always renews successfully at that moment.
What else can DHCP configure?
The address is one part of the binding. DHCP options can provide configuration such as a subnet mask, routers, DNS servers, domain names, and other parameters defined for the deployment. RFC 2132 defines the original DHCPv4 option set, and later standards add more options.
Do not assume every client requests, accepts, or applies every offered option. Capture the client's parameter request list, the server's response, and the effective host configuration. A valid address with the wrong gateway, DNS, route, or policy can still leave the device unable to reach the intended service.
How does DHCP work across subnets?
An initial DHCPv4 client may not yet have an address and commonly uses a local broadcast. Routers do not forward ordinary broadcasts between subnets, so larger networks use DHCP relay agents. A relay forwards client messages to a server and supplies link context that helps the server choose the correct address pool.
Document the client VLAN, relay address and policy, server destination, routing in both directions, firewall rules, and high-availability behavior. A reachable server can still offer from the wrong pool when relay or subnet selection is misconfigured. Treat relay metadata as operational input that must be protected from untrusted insertion or alteration.
Conflicts, security, and reliability
DHCP reduces manual configuration errors, but it cannot make an address plan correct by itself. Keep dynamic pools, reservations, exclusions, static assignments, and IP address management records aligned. RFC 5227 describes IPv4 Address Conflict Detection: a client can probe before use, send DHCPDECLINE after detecting a conflict, and continue to watch for conflicts while using an address.
Classic DHCP does not authenticate every server and client by default. A rogue or misconfigured server can provide a wrong gateway or DNS service, and resource-exhaustion traffic can consume pool or server capacity. Use authorized server and relay paths, access controls appropriate to the switching platform, monitoring, rate controls, resilient servers, configuration review, and an incident path. These controls reduce risk; they do not make all client identifiers trustworthy.
Does DHCPv6 use DORA?
No. DHCPv6 is a separate protocol defined in RFC 8415. A stateful DHCPv6 exchange commonly uses Solicit, Advertise, Request, and Reply messages rather than the DHCPv4 DORA names. DHCPv6 can provide non-temporary or temporary addresses, other configuration, and delegated prefixes; the exact result depends on the server and router-advertisement policy.
IPv6 hosts can also use Stateless Address Autoconfiguration (SLAAC). Router Advertisements, SLAAC, DHCPv6 address assignment, DHCPv6 information requests, and prefix delegation are related but distinct functions. Verify the intended IPv6 model from the router, server, and client state instead of copying an IPv4 assumption.
How to troubleshoot a DHCP lease
| Symptom | Check first | Common boundary |
|---|---|---|
| No offer | Client link, VLAN, broadcast path, relay, server availability, scope capacity, and policy | A running server is not proof that the client can reach the correct scope. |
| Wrong subnet or gateway | Relay link context, pool selection, reservations, option policy, and overlapping scopes | Renewing the same binding may reproduce the wrong configuration. |
| Address conflict | Dynamic pool overlap, static devices, stale reservations, client identifiers, and conflict logs | Removing one duplicate does not repair the source-of-truth error. |
| Lease will not renew | T1/T2, server identifier, routes, firewall, relay path, failover state, and packet timestamps | The device can work until expiration while renewal is already failing. |
| Address works but names fail | Offered DNS servers, search domains, client acceptance, route to resolver, and resolver health | Successful DHCP address allocation does not prove DNS works. |
Preserve a short packet capture and the matching client, relay, and server logs when policy permits. Record the observation time, transaction ID, client identifier, proposed and accepted addresses, options, and state transitions. Remove or mask personal device data before sharing diagnostic evidence.
DHCP lease FAQ
What is an IP lease?
In local-network administration, an IP lease usually means a DHCP binding that lets a client use an address and related configuration. Dynamic leases normally have finite lifetimes, while DHCPv4 can also represent an infinite lease. In an IPv4 marketplace, “lease” can instead mean a commercial contract for temporary use of a public prefix. Check which layer the speaker means.
Is a DHCP lease the same as leasing public IPv4 addresses?
No. A DHCP lease is a local configuration record. A commercial public IPv4 lease is a contract for time-limited prefix use and requires separate authority, registry, routing, reputation, operating, renewal, and return checks.
What are the four DORA steps?
Discover, Offer, Request, and Acknowledge are the common DHCPv4 messages used when a client obtains a new binding. Renewal, rebinding, decline, release, and other client states use additional exchanges.
What happens when a DHCP lease expires?
If a finite lease has not been renewed or rebound successfully, the client must stop using the expired address and restart acquisition. Existing applications can lose connectivity even though Wi-Fi or Ethernet still shows a physical link.
Will DHCP always give a device the same IP address?
No. A server may offer the prior address when it remains valid and available, but pool policy, lease history, client identifiers, reservations, topology, and server state can change the result. Use a documented reservation when local predictability is required.
Is a DHCP reservation the same as a static IP?
Both can make an address predictable, but they are configured differently. A reservation is selected and recorded by the DHCP server for a client identifier; a static address is configured directly on the host. Either method needs exclusions and conflict prevention.
Can DHCP work across VLANs or routed subnets?
Yes, normally through a DHCP relay agent or a server present on that subnet. The relay, routing, firewall, pool-selection policy, and return path must all be correct.
Does DHCPv6 use the same DORA process?
No. DHCPv6 uses different messages, commonly Solicit, Advertise, Request, and Reply for a stateful exchange. IPv6 can also use SLAAC, information-only DHCPv6, or prefix delegation according to the network design.
Primary DHCP sources
- RFC 2131: Dynamic Host Configuration Protocol
- RFC 2132: DHCP Options and BOOTP Vendor Extensions
- RFC 5227: IPv4 Address Conflict Detection
- RFC 8415: DHCP for IPv6
- RFC 4862: IPv6 Stateless Address Autoconfiguration
Choose the next address path
For a local network, start with the subnet, DHCP scope, relay path, address records, and client evidence. Read the IP address allocation guide and the public versus private IP guide for the surrounding model.
If the requirement is public IPv4 capacity rather than local host configuration, review the IPv4 leasing workflow or prepare a managed IPv4 leasing request.
