DHCP query relating to leases

Posted on 2008-10-04
Last Modified: 2012-06-21

I have a query with DHCP that I was hoping someone could help me with.

Let's say I have a subnet of

DHCP is configured to lease addresses from - The rest are static. The lease time is 7 days.

On Monday, I build 11 new machines that use up all the DHCP addresses.

I then take them offline and store them somewhere.

On Tuesday, if I place a new DHCP client enabled machine on the network, will it pick up an IP address from DHCP, or do I have to wait 6 days? Bearing in mind that the original 11 machines are offline, but still within their lease time.

Question by:kam_uk
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Expert Comment

ID: 22640080
Hi i will suggest you to hav a link of below mentioned link it may help you a lot in left side your screen you will find Lease Process .

LVL 58

Accepted Solution

tigermatt earned 200 total points
ID: 22640196
Hi there,

When DHCP leases addresses, it essentially removes those IP Addresses from the pool of available addresses which can be leased out, for the duration of the lease. Only when the lease time expires, and the client computer the lease was for has not returned to renew the lease, will the address be placed back into the pool of addresses available for leasing.

So, in your case, all the addresses from the pool have been leased for 7 days, so you would need to wait 6 days (or manually delete the leases in the DHCP console) before these leases are released and made available for other clients to make use of, again.

LVL 70

Assisted Solution

KCTS earned 50 total points
ID: 22640459
Yes you would, until the lease expires. the IP cannot be given to another computer.
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Author Comment

ID: 22640531
Hi, just to confirm, the DHCP server doesn't check to see if the original 11 machines are still connected? It will reserve their IP addresses regardless?

Author Comment

ID: 22640553
..and if we do intend to take the machines off the network, we should do an ip/release?
LVL 58

Expert Comment

ID: 22640619

That is correct. The DHCP Server has no idea that the machines have left the network, because a machine will not automatically release an IP address before it is switched off.

You can either ipconfig /release the machine before you take it off the network, or delete the IP lease from the Address Leases section on the DHCP Server. Only when an IP is not listed in Address Leases can it then be placed back into the pool of addresses for leasing again.

Of course, the other option is just to ensure your IP pool is large enough to have plenty of surplus addresses.

LVL 70

Expert Comment

ID: 22640665
No DHCP does not - and cannot check that the machines are still connected, until the lease runs out, their IP addresses cannot be assigned to another machine.

Yes you could to an ipconfig /release, bit if you have a big enough pool of addresses, it does not matter

The other alternative is to reduce the lease period so that the leases become available more quickly.

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