Periodic assignment of unique IP address

Simon Hobson dhcp1 at
Thu Nov 29 16:29:21 UTC 2007

Siraj Shaikh wrote:

>I have a few queries regarding the DHCP configuration

It might help if you described what it is that you wish to achieve, 
and these queries have all been answered fairly recently (though 
perhaps not in the exact words you have used).

>1) I am trying to set up my network, where on the same network segment
>I have two different sets of clients that are assigned IP addresses at
>different intervals. That is to say, the lease assigned to one set of
>clients is different to the lease assigned to the other set of
>clients. As far as I understand, I can do this by using DHCP user
>class mechanisms, whereby I deal with different sets of clients as
>part of different classes (and then each class is assigned a different
>lease time). Is that right?

Yes, you can treat different clients in different ways. You would 
normally need to use classes for this - lookup man dhcpd.conf and see 
the sections on classes and subclasses. If there is a characteristic 
of the clients you can match on then you can use a form of :
class .....
   match if <some condition is true>

You might do this, for example, if you wanted to select all VoIP 
phones from a particular vendor by the vendor prefix in the MAC 

If there isn't a matchable condition, then you may have to list each 
client by MAC address - see the subclasses section of the man page.

>2) Following up from the above, is there any way of assigning a random
>lease to a particular user class? Random in the sense that I would
>like to pass a range (upper and lower limits) of lease, and then let
>the DHCP Server assign a number in between randomly?

You mean, allocate a range of addresses for a class of client, and 
then the addresses are dynamically allocated from that range ? This 
is the default and is done like :

subnet ....
   pool {
     range a.b.c.10 a.b.c.29 ;
     allow members of "class1" ;

>3) My main question: how do I configure a DHCP client that allows it
>to be assigned a unique IP address everytime it renews its lease? Is
>it possible to configure the DHCP Server such that it assigns a
>particular class of clients (or just particular clients; manually
>assigned) a unique (different to prior) IP address every time their
>lease is renewed? I am interested to know the answer to both
>questions, as in from the configuration of a dhcp client and a dhcp

For both the server and client, this is specifically against the 
requirements of the RFCs which are quite clear that the client should 
retain it's address when it renews. Changing a clients address is bad 
news - it breaks all it's connections and causes all sort of grief !

What is the reason for wanting the client address to change ?

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