A lease specifies the amount of time the DHCP server permits a DHCP client to use a particular IP address. During the initial server configuration, you must specify a site-wide lease policy. The lease policy indicates the lease time and specifies whether clients can renew their leases. The server uses the information that you supply to set option values in the default macros that the server creates during configuration. You can set different lease policies for specific clients or type of clients, by setting options in configuration macros you create.
The lease time is specified as a number of hours, days, or weeks for which the lease is valid. When a client is assigned an IP address, or renegotiates a lease on an IP address, the lease expiration date and time is calculated. The number of hours in the lease time is added to the timestamp on the client's DHCP acknowledgement. For example, suppose the timestamp of the DHCP acknowledgment is September 16, 2005 9:15 A.M., and the lease time is 24 hours. The lease expiration time in this example is September 17, 2005 9:15 A.M. The lease expiration time is stored in the client's DHCP network record, viewable in DHCP Manager or with the pntadmutility.
The lease time value should be relatively small so that expired addresses are reclaimed quickly. The lease time value also should be large enough to outlast DHCP service disruptions. Clients should be able to function while the system that runs the DHCP service is repaired. A general guideline is to specify a time that is two times the predicted downtime of a system. For example, if you need four hours to obtain and replace a defective part and reboot the system, specify a lease time of eight hours.
The lease negotiation option determines whether a client can renegotiate its lease with the server before the lease expires. If lease negotiation is allowed, the client tracks the time that remains in its lease. When half of the lease time has passed, the client requests the DHCP server to extend its lease to the original lease time. You should disable lease negotiation in environments where there are more systems than IP addresses. The time limit is then enforced on the use of IP addresses. If there are enough IP addresses, you should enable lease negotiation to avoid forcing clients to take down their network interfaces when leases expire. If you make clients obtain new leases, the clients' TCP connections such as NFS and telnet sessions might be interrupted. You can enable lease negotiation for all clients during the server configuration. You can enable lease negotiation for particular clients or particular types of clients through the use of the LeaseNeg option in configuration macros.
Systems that provide services on the network should retain their IP addresses. Such systems should not be subject to short-term leases. You can use DHCP with such systems if you assign reserved manual IP addresses to those systems, rather than IP addresses with permanent leases. You can then detect when the system's IP address is no longer in use.