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 addressconfirm that it may continue using the address it was assigned.
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.
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