Doubts over DHCP Leasing way

Posted on 2009-04-23
Last Modified: 2012-05-06
Hi friends !

I am little bit confused about the DHCP leasing way.

For example, I have set DHCP lease for 15 days. Now a client gets an ip address from the DHCP server on 01st April 2009 at 8:00 a.m. (It means from 01st April to 15th April 2009)

It is obvious that client PCs are turned on in the morning and shut down in the evening when employees return back for home.

Now, my question is:

1. Does DHCP Server takes the ip address back from the client when the client pc is shut down and next day when the client PC starts again, it takes the new (or same) IP address from DHCP Server for next 15 days. (02nd April to 16th April 2009)


2. The same ip address is taken for 15 days and is not given to any other client PC in any condition.

Please also tell me what is good ? To have longer lease or shorter lease.

Question by:JatinHemant
    LVL 14

    Accepted Solution

    The following guide from Microsoft details how this process is handled -

    1. Once leased, both server and client will hold the leased address until the expiry of the lease. If the client connects to the same network before lease expiry, the server will just offer it the same IP without renewing the lease until the renewal time.

    2. It all depends on your network configuration. If you have enough IPs available to lease to all your users (desktops, laptops and VPN) at the same time, then you should leave the lease times at the current 15 days. However, if you have a lot of laptop and VPN users who are in and out of the network and you are running out of IPs to offer, then you should reduce the lease time to have more IPs available for users. If your user base is predominantly desktops, then leaving the long lease (15 days) will also avoid unnecessary broadcasts in your network.

    Hope this helps.

    Author Comment

    Thanks for your comment.

    Well...I need your suggestion about the leasing time. Here, I am working in an educational institute. We have arround 300 Desktop PCs and changing Wireless Clients (Laptops for students and employess).

    Right now, we are using / 16 (The default subnet mask). It gives a wider range that we CAN'T ever use.

    Someone told me that is is very BAD PRACTICE to use a wider range like / 16 (The default subnet mask) as there will be many many broadcasts in the network. And it is TRUE that we are also suffering from POOR network performance. (Internet speed is too too slow)

    I want to configure / 23 (means IP range for our network. It can accomdate 510 Hosts. I will reserve first 10 IPs for servers and rest 500 IPs will be for hosts.

    Tell me if it is OK. Will it be helpful to reduce the broadcasts ?

    We are planning to make VLANs but that is another issue.


    LVL 14

    Assisted Solution

    The formula I use is just double the maximum number of users. Chances are, you will probably end-up moving/upgrading the network before doubling the number of users anyway.

    Though broadcasts can cause poor network performance issues, it's more likely that a bad switch or some other client (Symantec Ghost for instance) is causing your performance issues and not the DHCP server. I would recommend you setup a monitoring PC with Wireshark or Microsoft Network Monitor and look to identify the origin of the broadcats.

    Author Comment

    I will surely go for Microsoft Network Monitor or Wireshart. Tell me which one is easier and easy to analysis.

    Well...I have setup the WINS Server and and DHCP Scope options to distribute WINS IP along with IP address, Subnet Mask, Gateway, ISP DNS Server's Addresses and one entry for P-node type 0x2 (Making all clients as P-nodes, so that they will directly contact WINS for name resolution)

    Please comment if it will help in reducing the broadcast.

    LVL 14

    Assisted Solution

    I recommend Wireshark as the interface is quite easy to work with.
    As for the node type, the standard practice is to use an h-node (0x8) which will query the name server first, then broadcast. But like you said, setting it to p-node will reduce the broadcasts.


    Author Comment

    Thanks for your support.


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