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DHCP Scope almost full

Posted by Joel-Pnlcpa
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Windows Server DHCP & IPAM

Working with Server 2019, DHCP scope of 192.168.101.x with a 255.255.255.0 mask. The leased range is 124 addresses, currently with around 42 actually used including a couple of reservations. My question is in event viewer I continue to see that 90% is in use and 9% is available. I've counted the leases and again it isn't even 50% used. I have restarted DHCP. As you can see DHCP skips almost every other address. I cannot find why this is happening or how to correct. So far I haven't reached the full threshold. 


Spice (17) Reply (12)
Joel-Pnlcpa

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  • I don't know where the "124 addresses" comes from (or the "9% available").

    You said the mask is 255.255.255.0 so that allows for 254 addresses. The screen shot shows addresses from .8 to .250 so that's a spread of at least 242 right there.

    With only 50 devices on the network I wouldn't worry about it.

    Of course, if you have more devices, ones that you perhaps don't know abut, then you might worry.

    Devices with two network cards (laptops) might use more addresses than you expect. Smartphones, smart watches, tablets etc may be connecting. Someone may have shared your WiFi network key publicly so you have more connections than expected.

    Also count your managed switches, routers, firewalls, servers, network connected UPSes, security systems (cameras and door entry controls), thermostats, printers, etc.

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    • What is your lease time?  You could try decreasing this as a lot of IP addresses grabbed sometimes are no longer needed but they won't release until the lease time is up.

      Mine is set to 7 days.

      Spice (5) flagReport
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    • If the screenshot was taken "today" then your DHCP lease expirations appear to be somewhat long, which is most likely why the address pool is filling up.

      Unless there is a specific reason why it is set to 4 days you can change the client lease length to 24 hours and the list of used IP's will become shorter without introducing too much additional network traffic. Keep in mind that this change will apply to new address leases, the existing leases will still only expire at the designated time unless you manually delete the entries (not advisable).

      The skipping of IP's is normal behavior, it could be leases that expired and wasn't renewed because the device is offline or it got another IP address for whatever reason.

      Spice (2) flagReport
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    • I also was going to suggest shortening the lease time. In addition, something isn't adding up with your explanation. You say there are only 124 addresses in the scope, but the subnet allows for .1-.254. Are you excluding addresses from the range? Have there been recent changes to the DHCP scope settings? Is there anything configured in the Policies section (e.g. for VoIP phones)?

      Even though the DHCP service was restarted, have you restarted the server entirely?

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    • I don't know where the "124 addresses" comes from (or the "9% available").

      You said the mask is 255.255.255.0 so that allows for 254 addresses. The screen shot shows addresses from .8 to .250 so that's a spread of at least 242 right there.

      With only 50 devices on the network I wouldn't worry about it.

      Of course, if you have more devices, ones that you perhaps don't know abut, then you might worry.

      Devices with two network cards (laptops) might use more addresses than you expect. Smartphones, smart watches, tablets etc may be connecting. Someone may have shared your WiFi network key publicly so you have more connections than expected.

      Also count your managed switches, routers, firewalls, servers, network connected UPSes, security systems (cameras and door entry controls), thermostats, printers, etc.

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    • Lease time is 4 days. The server is rebooted on Saturday morning each week. Haven't rebooted it yet. There are a fair number of exclusions 1-60, 170-254 for servers and other static devices. 

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    • All other managed devices, IoT devices, etc. are statically assigned in the exclusion ranges. Wireless is on a different subnet entirely. 

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    • Is there a failover partner DHCP server in the equation? I'm only familiar with failover (as opposed to load balancing) ... but a failover partner always has a percentage of the pool reserved for its own use if the primary goes offline.

      Spice (1) flagReport
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    • No failovers or load balancing. Single DHCP server.   

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    • If 1 to 60 and 170 to 254 are reserved, you only have 110 addresses to dynamically assign.

      I see .8 is reserved, but it still gets assigned (as far as the server is concerned.

      Are you sure about the exclusions? Because 110 <> 124.

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    • The full scope is 1 - 254 minus 1-7, 9-60, 179-249 for exclusions, also minus 8 since it is a reservation. My concern is that it appears to be skipping addresses throughout the scope. 

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    • I performed a reconcile/verify and found all of those unused address assigned to GUIDs, not MAC's, that I can't find apply to anything. Removed them (hopefully not detrimentally) and now it is down to 33% in use. 

      Spice (1) flagReport
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    • Set your lease time to 8 hours. That will free up ips. If you are still low, you can go to a /23. 

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