One of the major advantages offered by DHCP is the ability to lease an IP address, meaning assigning it on a temporary basis. Usually, when the client leaves the network, that particular assigned IP address is then free to be allocated to another device by the DHCP server.
DHCP leases are related to every DHCP allocation and define for how long a user is allowed to use an allocated IP address. This parameter is usually administratively configured inside the DHCP scope. Whenever a client is rebooting, that client will again have to ask the DHCP server for an IP address. The DHCP server is usually configured to re-allocate the same address and extend the lease for that specific client.
Workstations can also manually release the IP address, for example, in these situations:
- The device is turned off indefinitely
- The device moves to another subnet (e.g., to a wireless network from a wired network)
The leasing process has a number of timers associated with it, so you can be sure that you will always have an IP address that is updated on every network device. There are two important DHCP timers, as follows:
- The T1 renewal timer (default: 50% of the lease time): Whenever a workstation obtains an IP address, this timer starts up and when 50% of the lease time has been reached, the DHCP client will try to renew its lease with the original DHCP server.
- The T2 rebinding timer (default: 87.5% of the lease time): This second timer is used in situations in which the DHCP server does not answer or confirms the allocation extension after the renewal timer expires. This timer states that if 7/8ths of the lease time has passed, the client will try to find (by sending a DHCP Request) other DHCP servers that might be able to provide a DHCP address.
With the lease process in place and correlated to the timers presented above, you can be sure that you will always have an IP address in a timely manner without any downtime associated with this; in addition, you automatically have a way to build redundancy into the DHCP process. The T1 and T2 timers are presented in relation to the lease time in Figure 10.2 below.
Figure 10.2 – DHCP Lease Timers