What is DHCP Lease?

6 September 2019

EfficientIP

A DHCP lease is a temporary assignment of an IP address to a device on the network. When using DHCP to manage a pool of IP addresses, each client served on the network is only “renting” its IP address. Thus, IP addresses managed by a DHCP server are only assigned for a limited period of time.

The period of validity of the assignment is called a lease duration and when it expires, the client shall immediately stop using this IP address and stop all communication on the IP network. The main risk of not complying to this rule is to have more than one device on the network using the same IP address with conflicts on delivering IP frames to the right device (aka duplicate IP address).

DHCP lease duration is expressed in seconds. It can be specified as “infinite” for permanent leases, usually used for devices that should not change their IP address without a need to change their configuration (some IOT, printers or application servers).

Depending on the lease duration, clients on the network may need to continue using the same IP address, therefore need to extend the lease period. Half way through the lease time (T1 timer), the client can contact its DHCP server in order to extend its lease period via a lease renewal request. This renewal process can be performed multiple times if no answer from the server has been received. If for any reason the renewal of the lease hasn’t worked by 7⁄8 of the way through the lease duration (T2 timer), the client can try rebind its lease via a broadcast to any DHCP server on the network. If the whole process has been unsuccessful before expiration of the lease period, the client should stop using its IP address and go through the whole process from the beginning.

During the DHCP lease period, the client can ask the server for a lease termination, in order to free the IP address so that it can be used for another client on the network. This process is generally automatically performed when a host shuts down.