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recommendations for setting the lease time on a dhcp server

Hello guys.  I'm new to this organization and when I checked the settings for the leases on the dhcp servers, they are set to 999 days.  Not sure what information you need so I'll provide the basics about this organization.  I work in a school setting with approximately 20 schools.  Each school exists in its own site.  The elementary schools, for example, probably have approximately 150, 250 desktops and laptops.  There's not much addition or subtraction to these numbers.  With this information and I hope it was sufficient, what would you guys configure for the leases.  One of the administrators seemed to think 999 days was a terrible configuration.  Anyway, I value the opinions I get on this site so I come here for input.  Thanks as always for your expertise.
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Mal Osborne
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Malmensa, I just want to be clear on something you said.  If there's a pc on the network and under the current configuration with the lease being 999 days and that pc is disconnected from the network, that PC will maintain that lease for the full 999 days unless someone manaually deletes the connection or the lease expires.  If that's the case, I definitely need to change the lease durations on the dhcp server.  I just want to be sure on that.  Thank you for your response and expertise.
Long lease times are a problem if you have to make a change to the DHCP scope - a change to a DNS or WINS server address, perhaps. If the lease time is 8 hours, then all your desktops will pick up any changes made by the time work starts the following day.
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I'm sorry meyersd - I screwed up your name.  I don't know what the hell I was looking at.  I'm definitely going to follow your advice and make the change to the lease durations.  In the scenario you pointed out with changing the scope and the long lease times, could you delete the existing leases if you were in a situation like that or would you have to reboot all the clients so they would have to renew the ip settings.  I know when we changed the scope once, I had to go around and reboot all the clients.  It was definitely a pain and your lease duration recommendation would have prevented that problem.  Thanks again.
It's going to be a bit of a pain. You'll have to make the changes at the DHCP server the manually release the DHCP lease on all the workstations.   :-(
pendal1:  Depends, the DHCP server will keep that lease for 999 days, after it has been renewed. The workstation will attempt to renew once the least is half finished.  ie, After 499.5 days, it will renew, & reset to 999 days.  A lease will take between 499.5 & 999 days to expire, once the machine has been removed.  That is a very long time!
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Malmensa, that is good to know.  So the workstation, if connected to the network, will attempt to renew at the halfway mark to live out the lease.  However, if the workstation is disconnected from the network and doesn't have an opportunity to renew at the halfway mark, the pc looses the lease and that ip address goes back into circulation.  I hope I have that correct.  What would you recommend for the lease duration?  I have an 8hr and a 30 day recommedation so far from meyersd and gmwalden?  
gmwalden, what do you mean when you say you can use dhcp to loop back if there is a problem?  As always, thanks to everyone for your time and expertise.
Sorry, look back.  I just give you a way to see what machine has what ip address.  This can be useful if a machine is powered off, and you need to know something about its IP, MAC, or computer name.  Depending on how your network is setup, it can help you pen down a computer.
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Thank you very much gmwalden.  Guys, one more question.  If I change this lease duration on the server from 999 days to let's say 8 days, etc, and apply the change - when will this change in lease duration kick in.  Do i have to reboot the machines or anything like that.  Thank you.
Yes if you make the change it will kick in.  No rebooting should be needed.  Although i would recommend rebooting the PC daily before the user leaves to go home.
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Thank you all for your time, consideration, and expertise.  Greatly apprecaited.