TITLE
    TCP/IP Options: What does "Don't retain DHCP lease on Shutdown" do?
Article ID:
Created:
Modified: 		30994
5/19/99
3/14/00
TOPIC

    This article explains the effect of the "Don't retain DHCP lease on Shutdown" checkbox in the TCP/IP Options control panel, and explains why users may want to use it.

DISCUSSION

    Note: A detailed technical explanation the DHCP client, including this option, may be found in:
    Tech Info Library Article 58372: " Open Transport: DHCP Client Behavior "

    By default, if a Mac running Mac OS 8.6 is shut down (or put to sleep, in the case of PowerBooks), it does not let the DHCP server know that it no longer needs the assigned IP address. Then, if the computer is booted (or a PowerBook is woken from sleep) before the lease expires, it will remember the last IP address and request to use it again.

    One effect of this is that leases may be left dangling - meaning that even though the original client is not using the IP address, the server cannot give it out to another client which needs it since it wasn't explicitly released. Normally this is not a problem and the administrator can configure the DHCP server for an appropriate lease time.

    By checking the box for "Don't retain DHCP lease on Shutdown" you are telling the client Macintosh to always release the IP address when the Mac is either shutdown or restarted. Then on next startup, the Mac will configure DHCP as if it were starting up for the first time. This may result in the Mac receiving a different IP address from the server.

    There are a couple of situations where this may be preferable:

    1. If there is an extreme shortage of IP addresses for the network and administrators want them released the instant they are not being used.

    2. For computers that frequently move from one physical network to another. This is particularly relevant to PowerBooks. This way, you can be sure that (a) the IP address is released when the computer is no longer on the network and there is no chance the client will need it again, and (b) when the PowerBook boots or wakes up, it won't assume that it is on the same network it was on when it was shut down or put to sleep.

    The default DHCP behavior for Mac OS 8.6 should provide optimum performance for most situations. Unless you are encountering one of the specific situations mentioned above, or unless you are specifically directed to do so by your network administrator, Apple recommends that you not alter the default 8.6 behavior by checking this box.

    Note: There is currently an issue with the TCP/IP Options control panel that will not allow you do de-select this option once it is turned on. For a full description and work-around, see:
    Tech Info Library Article 58375: " TCP/IP Options: Can't Turn Off "Don't Retain DHCP lease "

    If you have a CD with Mac OS 8.6 on it, the TCP/IP Options control panel may be found in the Network Extras folder. Otherwise, it is included in Open Transport Extras 2.0.3, available at Apple Software Updates:

    http://asu.info.apple.com/swupdates.nsf/artnum/n11401

Document Information
Product Area: Communications-Networking
Category: LocalTalk-EtherTalk-Token Ring
Sub Category: General Topics

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