DHCP Lease Period
n The server must choose an expiration time for the lease, as follows:
IF the client has not requested a specific lease in the DHCPDISCOVER
message and the client already has an assigned network address, the server
returns the lease expiration time previously assigned to that address (note that
the client must explicitly request a specific lease to extend the expiration time
on a previously assigned address), ELSE
IF the client has not requested a specific lease in the DHCPDISCOVER
message and the client does not have an assigned network address, the server
assigns a locally configured default lease time, ELSE
IF the client has requested a specific lease in the DHCPDISCOVER message
(regardless of whether the client has an assigned network address), the server
may choose either to return the requested lease (if the lease is acceptable to
local policy) or select another lease.