Dhcp assigns duplicate IP

Hello I always have the problem that an ip is assigned in the dhcp already assigned to another client. The other client is offline at the time of dhcp allocation.

Is this a bug or am I doing something wrong here in the settings?

Hi @anwin ,

I suspect that your problem will be that your IP Address that is assigned to another client is in the Dynamic Range of to

Assigned Fixed IP Addresses must not be from within the dynamic range. This is mentioned in the Wiki page on DHCP.


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thanks for that, who can read is clearly at an advantage.


Short description of IP allocation of dhcpd:

  • Is the MAC defined in a host definition ( fixed leases )? If yes, choose the IP of the definition.
  • If no, choose an IP out of the dynamic pool ( the range defined by start and end address ), check if host in the network responds to this IP. If not, allocate this IP to the MAC. If yes, choose another IP and test.

I hope this makes it a bit clearer, why the fixed leases must be chosen outside the dynamic pool.

I had not read through the doku because I could assign from windows DHCP simply this assigned IP and that was then also no longer assigned, is also with the Fritz so if I am not mistaken.
But well I have now changed the DHCP range afterwards and will see if that works now.

If you read through the ISC DHCP Server documentation and search their mailing list you will find that they say that you should not use an IP Address from the Dynamic Range for use on a Fixed IP definition.

The response is that it might work, depending on lease times etc, but it also may not work if fixed clients are disconnected for some time and other clients come on stream.

The fact that systems allow this to happen does not make it right.


I got it, all good

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