Clarification on fixed-address statement

Jack Kielsmeier jackkiel at
Fri Apr 29 21:08:52 UTC 2011

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

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: 

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

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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