What is a DHCP lease?
DHCP does not give an address away permanently. The server lends it for a set period, the lease duration, and records the client's MAC address (or client ID), the address, and when the lease ends. While the lease is valid, the server will not offer that address to anyone else. If the client leaves and never renews, the lease runs out and the address goes back into the pool.
Lease lifecycle
- Obtain: the client gets the lease through the four-step DORA exchange (Discover, Offer, Request, Acknowledge).
- Renew at T1 (50% of the lease): the client sends a unicast DHCPREQUEST to the server that issued the lease. A DHCPACK resets the lease clock and the client keeps the same address.
- Rebind at T2 (87.5% of the lease): if the original server has not answered, the client broadcasts a DHCPREQUEST that any DHCP server can answer.
- Expire: if no server answers by the end of the lease, the client must stop using the address and start over with a DHCPDISCOVER. A Windows client that still finds no server assigns itself an APIPA address (169.254.x.x).
- Release: a client can hand the address back early with a DHCPRELEASE message.
Example: with an 8-day lease, the client tries to renew after 4 days and starts rebinding after 7 days.
Choosing a lease length
| Short lease (hours) | Long lease (days) |
|---|---|
| Addresses return to the pool quickly | Fewer renewals and less DHCP traffic |
| Suits guest Wi-Fi, cafes and conference rooms with high turnover | Suits offices where the same desktops connect every day |
| A DHCP server outage affects clients sooner | Devices that left still hold addresses until their leases run out |
Windows Server DHCP defaults to an 8-day lease for new scopes. A small scope with many visitors and a long lease is a common cause of address exhaustion.
Viewing and refreshing a lease
ipconfig /all(Windows) shows DHCP Enabled, DHCP Server, Lease Obtained and Lease Expires for each adapter.ipconfig /releasesends a DHCPRELEASE and clears the adapter's IPv4 address.ipconfig /renewasks for a lease again; after a release it runs the full DORA exchange. The IPv6 versions areipconfig /release6andipconfig /renew6.- On many Linux systems,
dhclient -rreleases the lease anddhclientrequests a new one; systems managed by NetworkManager can reconnect the interface withnmcli. - On the server, the Windows DHCP console lists active leases under Address Leases, and a lease can be turned into a reservation from its right-click menu.
Troubleshooting cues
- A laptop moved to a different network or VLAN keeps its old address: release and renew.
ipconfig /allshows a 169.254.x.x address and no DHCP server: the client never got a lease. Check the cable or Wi-Fi association, the DHCP server, the scope and any relay agent.- A device's address keeps changing and breaks printing or a port forward: give it a DHCP reservation or a static address outside the scope.
- The pool fills up with devices that are no longer present: shorten the lease duration.
DHCP lease on the CompTIA A+ exam
Leases is a bullet under DHCP in Core 1 objective 2.4. Know that a lease is temporary, that renewal starts at 50% of the lease and rebinding at 87.5%, and which ipconfig switches release and renew it. A scenario that ends with a 169.254.x.x address points to a failed lease request, not a bad lease length. A question about a device that needs the same address every time is about reservations, not lease duration. If a question asks what happens halfway through a lease, the answer is that the client tries to renew with its original server.