Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

With ISC DHCPd under Linux, I can define separate pools for "known" and "unknown" clients, like this:

subnet netmask {
    option routers;
# Unknown clients get this pool.
    pool {
        option domain-name-servers;
        max-lease-time 300;
        allow unknown-clients;
# Known clients get this pool.
    pool {
        option domain-name-servers,;
        max-lease-time 28800;
        deny unknown-clients;

host SomeHost1 { hardware ethernet 00:0F:1F:BC:A0:B9 ; }

By not specifying an IP for the host, SomeHost1, the DHCP server figures it out. This is important to me, as I have a DHCP server configured to support several VLANs (with DHCP relay on the layer-3 switches).

How can I use such a configuration with Microsoft's DHCP server under Server 2008 R2?

This is also important as some clients may move from one VLAN to another (if they used wireless and went to another building for example). I cannot simply make reservations for each MAC, as then it would no longer be valid if they moved to a different VLAN.

share|improve this question

"The unknown-clients flag is used to tell dhcpd whether or not to dynamically assign addresses to unknown clients. Dynamic address assignment to unknown clients is allowed by default. An unknown client is simply a client that has no host declaration." -

In Windows DHCP server, this concept is called Classes:

Does this work for you?

share|improve this answer
Unfortuantely, I don't think that will work. I need to be able to provide a list of addresses that end up in one pool, and all others end up in another. – Brad Jul 25 '11 at 18:23
What do you mean "provide a list"? To what are you providing a list? Correct me if I'm wrong, but your trying to actually figure out how to qualify which clients are part of "unknown-clients" in the ISC DHCPd configuration. The answer is above. The method to qualify clients as "unknown-clients" in Windows DHCP server is the method described above. – mbrownnyc Jul 25 '11 at 18:46
How do I assign the pool members without putting in a reservation for a specific IP? ISC DHCPd decides known/unknown clients by whether or not they are listed as a host in the config file. See the last line of the configuration I posted. Basically, the config file will have thousands of these, and they get put into the known pool. All others are put into the unknown pool. – Brad Jul 25 '11 at 18:48
Well, how do you determine which DHCP ruleset gets given to the client? Do you want to qualify the clients by MAC or do you not? If not, how do you want to qualify the clients? – mbrownnyc Jul 25 '11 at 18:53
I have a captive portal (modified Untangle) that provides a list of clients by MAC. Prior to the the client registering with the captive portal, they will be in one address pool. After they register, they should be placed in a new pool. The reason is that I can configure the portal to ignore a "registered" pool so they don't have to re-register every morning. I planned on simply writing a script to add reservations for everyone, but ran into the problem where they can move to a different VLAN, and thus need a different IP address. – Brad Jul 25 '11 at 19:01

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.