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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Configuration and Operation

DHCP Lease Allocation Process 1 2 DHCP Lease Renewal and Rebinding Processes

When a DHCP client starts up for the first time and has no lease, it begins in the INIT (initialize) state and goes through the allocation process described in the preceding topic to acquire a lease. The same process is used when a lease ends, if a lease renewal fails, or some error or other happening causes a client to need a new lease.

There are certain situations in which a client starts up while it still has a lease already in place. In this situation, the client does not need to go through the entire process of getting an IP address allocation and a new lease setup. Instead, it simply tries to re-establish its existing lease, through a process that I call reallocation.

There are two primary circumstances under which a client performs reallocation rather than allocation:

• Power On With Existing Lease: The length of time that a client lease lasts can range from minutes to years; it is entirely a matter of the lease length policy set for the network and client by the administrator. Many, if not most client machines are not left connected to the network 24 hours a day; they are turned on during the day and then shut down at night, and also shut down on weekends. A client with a very short lease that is shut down and then later started again will probably find that its lease has expired, and will have to get a new one. However, if a lease is longer than a few days, it will still probably be in effect when the client starts up again.
  
  
• Reboot: Clients are also sometimes rebooted, to install new software or correct a problem. In this case even when the lease length is very short, the restarting client will still have a valid lease when it starts up.

DHCP Lease Allocation Process 1 2 DHCP Lease Renewal and Rebinding Processes