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Table Of Contents  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 Address Assignment and Dynamic Address Allocation and Management

Previous Topic/Section
DHCP Address Assignment and Allocation Mechanisms
Previous Page
Pages in Current Topic/Section
Next Page
DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers
Next Topic/Section

DHCP Leases, Lease Length Policies and Management
(Page 4 of 4)

Factoring Lease Renewal Into Lease Length Selection

In selecting a lease time policy, the administrator must also bear in mind that by default, after half the length of a lease, the client will begin attempting to renew the lease. This may make it more advisable to use a longer lease time, to increase the amount of time between when a client tries to renew the lease and when the lease expires. For example, in a network with a single DHCP server, an administrator may want to use leases no shorter than eight hours. This provides a four-hour “window” for maintenance on the server without leases expiring.

When a lease is very short, such as minutes or hours, it will typically expire when a client machine is turned off for a period of time, such as overnight. Longer leases will persist across reboots. The client in this case will still contact the DHCP server each time it is restarted to reallocate the address—confirm that it may continue using the address it was assigned.

Key Concept: A key decision that a network administrator using DHCP must make is what the network’s lease length policy will be. Longer leases allow devices to avoid changing addresses too often, while shorter leases are more efficient in terms of reallocating addresses that are no longer required. An administrator can choose from a variety of different lease times, and may choose longer leases for some devices than for others.

Issues With "Infinite" Leases

In addition to choosing a particular lease length number, it is possible to specify an “infinite” lease length duration for certain clients. This effectively turns dynamic allocation into automatic allocation for a particular client. As I said in the previous topic, however, this is generally not done. The reason is that an infinite lease never expires, and as the old saw goes, “never is a long time”. J

Permanently assigning an IP address from a pool is a somewhat “risky” move, because once assigned, if anything occurs that causes that address to be no longer used, it can never be recovered. A worst-case scenario would be a visitor to a company site who plugs a notebook computer in to the network to check e-mail or transfer a file. If that machine is assigned an IP address using automatic allocation, the visitor will “take it with him” when he leaves. Obviously, not a great idea.

For this reason, most administrators prefer to use dynamic allocation instead, with addresses set to a very long time frame, such as a year or two years. This is considered “near enough to infinity” that it approximates a permanent assignment, but allows an IP address to eventually be recovered if a device for whatever reason stops using it. In such a policy, anything that really, truly needs a permanent assignment is given an address using manual assignment, which requires a conscious decision to dedicate the address to a particular device.

Related Information: For a little more information related to lease length selection, see the topic on DHCP server implementation problems and issues.

Previous Topic/Section
DHCP Address Assignment and Allocation Mechanisms
Previous Page
Pages in Current Topic/Section
Next Page
DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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