Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

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

DHCP Address Assignment and Allocation Mechanisms 1 2 3 4 DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers

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.

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.

DHCP Address Assignment and Allocation Mechanisms 1 2 3 4 DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers