Dhcp server leases and their behaviour

  • I'm experiencing the number of dhcp leases reach the maximum allowed on the range thus no more available for further clients.
    The leases are mostly "offline"
    Default lease time is set to 86400 (perhaps wrong)
    Maximum lease time is blank

    Is there anyone explaining me how dhcp server works with leases and these timers ??
    What are the suggested values ?

    Thank you very much

  • It's basically what it says.

    Default lease time is what  clients get when they don't request a longer or shorter time

    If you have many devices that only connect for short periods then you can put max at 7200(2h)en default at 3600(1h)

  • But, what happens to client side when lease time expires ??
    Let's say I'm using my device and lease time expires, do I loose connectivity while device re-negotiates a new ip address ??
    Or lease expires only when device is inactive ?

    Thank you

  • Devices apply for a new lease right before the lease expires. (Renew)
    This means there is no connectivity loss.

  • If a client doesn't have an IP address it will send a DHCP discover, this is a broadcast message.
    The server will reply with a 'DHCP offer' this is also a broadcast with an IP address that the client can use.

    The client will then send a 'DHCP request' with the IP address offered by the server. Again this is a broadcast frame since the IP address still isn't comitted to this client.

    As an answer to this request the server can send either a 'DHCP ack' message telling the client that the IP address is committed and the client can use it.
    Or the server can send a 'DHCP nack' denying the requested IP address and then the whole story starts over again.

    If thc client has its IP address it will send a 'DHCP request' to the server to prolong the usage of the current IP address. This happens when half the lease time has expired.
    The server can reply with an ACK, keep using it, or with an Nack, release the IP and start requesting a new address.

    Finally, the client can send a 'DHCP release' message to the server telling the server to release it and he can use it for another client.

    Usually this doesn't happen very often. The client gets powered down and the lease time will expire.
    When the client starts up again it will send a 'DHCP discover' which the server recognizes as coming from the same mac address as before and it will hand out the same IP adress.

    If there is a DHCP forwarder in between the procedure is a bit different on the server and forwarder side. For the client it remains the same.


  • Thank you both for clarifying  !