Can DHCP be made to force the use of a different IP for each lease?

Posted on 2012-09-14
Last Modified: 2012-09-14
I am not up on the various DHCP implementations and the tricks with them, does anyone know if there is a way to have DHCP give out a *different* IP each time, or at least a very good chance?
Question by:docsteel
    LVL 76

    Accepted Solution

    Depending on your lease time as well as the number of IPs in your dhcp scope.
    While the systems are up, they will renew the IP they were originally assigned.

    Note that a short lease time will mean there will be a larger number of DHCP packets consuming the bandwidth as well as should an issue come up with the DHCP server, the systems might become unusable once the lease expires.

    Author Comment

    Thanks Arnold - I thought about a shorter lease time - but I would think the DHCP server would just keep giving the same IP out to the same client unless it was exceptionally low and the number of IP's was slightly too small.  What would you do in terms of lease time and IP number to increase the odds that a different IP is given out?
    LVL 76

    Assisted Solution

    What is the purpose for giving different IPs to the same system?
    Usually, having the same IP assigned to the same system simplifies identifying issues.
    i.e. if you have a proxy setup, an attack on your internal systems will speed up the identification of the source by IP alone.

    without a lease expiring and the prior holder of the IP being down, the IP will not be assigned to a new system.
    i.e. a pool of 10 IPs and 10 systems will tend to get the same IPs unless systems remain down for sometimes and are regularly restarted.

    get wireshark from or use microsoft network monitoring tool to capture DHCP discover/offer/renew packets to determine whether what you want to achieve is worth the effort that you have to put in.

    if you know for certain that systems are shutoff after a certain time of the day, you might be able to use scripting tools, wmi, vbscript to clear the lease table. This way when the systems start, they will be more likely to be assigned a "new" IP based on their start order.

    Author Comment

    I realize it's a strange request - in the full context of the situation I am dealing with it is the desired option.  Thanks for all the help guys, I think it's clear that DHCP doesn't have any backdoor methods to accomplish this, outside of tinkering with pool size and lease time.
    LVL 76

    Expert Comment

    Have not looked at it directly, but if you are open to using non Windows Based DHCP server, you could look at whether a linux/unix based DHCP could be setup and use an external program that will act as a check on whether an IP should be issued to a system (based on MAC address).
    LVL 77

    Expert Comment

    by:David Johnson, CD, MVP
    note: all computers know their last ip address, and will ask for this ip address if available the dhcp server will give it to them

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