DHCP lists a false negative - I want to undertstand how to correct it


We have a small company that has a DHCP server that hands out private IP Addresses.  The scope is from .100 - .175 (76 total IP addresses).

I saw that we only had 5 available IP addresses to give out yesterday so I began re-configuring some client devices from using DHCP settings to use Static IP addresses that are outside the DHCP range.  I was able to free up 18 IP addresses last night.

From yesterday (shown below),

After I made a few changes (shown below),

My question is, why is our DHCP server showing 33 used IP addresses (shown below)

but, the display statistics show 55 are in use.  Why are the numbers not adding up?
PkafkasNetwork EngineerAsked:
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JohnBusiness Consultant (Owner)Commented:
You likely have devices off or disconnected for which the DHCP server is still reserving the lease. If it is important to show just what is connected, try reducing DHCP lease times.

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Jason JohanknechtIT ManagerCommented:
I would start by figuring out what is getting IP address from DHCP.  Do you have WiFi and allow everyone to connect smart phones?  Is the WiFi encrypted to prevent others from connecting?  Do you have Dynamic IP network printers?  How many actual computers do you have?  Also make sure if you have wired connections to laptops, that they aren't also connecting via WiFi.
Jason JohanknechtIT ManagerCommented:
John is right on the DHCP lease times.  If you have alot of changing clients like smart phones, you should lower that setting.
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Fred MarshallPrincipalCommented:
I agree you should check the lease time setting on the DHCP server.  The question remains: "what is a good lease time?".
I generally use 8, or perhaps 12, hours.  I choose this because:

Re-establishing a lease is transparent to the user and doesn't take much communication to happen.  So why not do it frequently?   Surely there's a balance between too frequently and too infrequently.

When a client has a lease, it's supposed to renew the lease when the lease time is half over.  So, an 8-hour lease would be renewed after 4 hours and after 4 hours again and so on ..... This tends to stabilize the IP address assignment as the renewal doesn't change the assignment.

When a lease expires, the address becomes available.  A lease will expire if the device is no longer on the network and the lease time is up. If there's some added margin in this time, I don't know of it.  (i.e. if a lease expires, is the IP address/MAC address relationship retained for even a longer time?).  Otherwise, what does a lease expiring really mean??

When there are multiple mobile devices coming in and out of the site, shorter lease times allow for their assigned addresses to be freed up and available for other devices coming in.  

If I had chosen 2 hour or 4 hours then I doubt there'd be any noticeable difference - except that addresses would be freed up more often.

You can have something like the "best of both worlds" by assigning IP/MAC relationships inside the DHCP server settings.  Then the addresses will appear as if they were *static* but the clients will receive updated gateway and DNS addresses as they occur or if they occur.  But, of course, the list needs some tending over time.
PkafkasNetwork EngineerAuthor Commented:
it sounds as if shortening the lease time is the way to go.  Currently, the lease time is set to 2 days.

If I change that to under 24 hours, will that negatively affect any users that are active on the system?  I am just trying to think if there will be any side-affects?

Perhaps I should wait for the weekend to change the lease time, when no one is in the office, what do you think?
JohnBusiness Consultant (Owner)Commented:
If I change that to under 24 hours, will that negatively affect any users that are active on the system?

No. No impact if the system is ON and connected and likely they will retain the same internal IP next day. No impact or harm if they shut down and get a different IP.
JohnBusiness Consultant (Owner)Commented:
@Pkafkas  - Thanks, and I was happy to help.
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