Clarification on fixed-address statement

Sten Carlsen stenc at
Fri Apr 29 22:36:41 UTC 2011

Correct, dynamic ranges can not overlap fixed addresses without issues
like what you see.

As mentioned they are handled in different parts of the code and the two
parts of the code do not communicate that information. So the code for
dynamic ranges will hand out what you have given them and the fixed
address code will also hand out what you gave them.

We could wish for changes or at least warnings but this is what we have
until "somebody" gets the time and money to make changes and then it is
a question how things could/should be changed. So far no changes I heard of.

I don't write the code, so others will have to comment on that.

On 29/04/11 23:08, Jack Kielsmeier wrote:
> We do have static ranges, yes. But some of the fixed addresses, for whatever
> reason, fall within a dynamic range. I know this is far from optimal, but
> it's how things presently are set up.
> So, what I'm hearing here, is that there is no way to restrict handing out
> an IP address to a certain MAC, if that IP address is part of a DHCP pool
> statement?
> That makes sense to me, just asking for clarification.
> Thank You,
> Jack Kielsmeier
> netINS Systems Administrator
> systems at
> ________________________________________
> From: at
> [ at] On Behalf Of
> Sten Carlsen
> Sent: Friday, April 29, 2011 3:51 PM
> To: dhcp-users at
> Subject: Re: Clarification on fixed-address statement
> You do keep the fixed addresses in a different range than the dynamic?
> A fixed address statement does not prevent the same address being handed out
> as dynamic if it lies within that range.
> On 29/04/11 22:06, Jack Kielsmeier wrote: 
> Hello,
> We are running ISC DHCP 3.1.0 in a failover configuration. I do know
> that 3.1.0 is a somewhat old version, but it has worked well for us.
> I am wondering if this is the expected behavior of fixed-address
> statements:
> Upon initial inspection, it seems that if you declare a fixed-address
> for a MAC address, that IP address is reserved and can only be used by
> the declared MAC address.
> In reality, we are seeing the DHCP server hand out fixed-addresses to
> any MAC. When the MAC address that is assigned the fixed-address asks for
> an IP, DHCP will attempt to give it the fixed-address if it is free, but
> will hand out another IP if it's unavailable.
> Is this how fixed-address statements are supposed to work? If so, is
> there a way to make sure only a specific MAC address can use a specific
> IP address?
> For reference, this is how we are declaring fixed-addresses:
> host HOSTX { hardware ethernet xx:xx:xx:xx:xx:xx;fixed-address
> ###.###.###.###; }
> I have stripped the MAC and IP address out.
> Thank You,

Best regards

Sten Carlsen

No improvements come from shouting:


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