DHCP Scope and re-assignment WS 2008R2

Posted on 2011-10-12
Last Modified: 2012-05-12
We have a windows server 2008 r2 with dhcp and everything setup internally for single class c network, with G/W (cisco router) We need more IPs for our internal network.

What is the best way to add more IPs for the internal network - without having to reconfigure all the existing computers?
Question by:freshgrill
    LVL 14

    Accepted Solution

    Note that the range have 254 usable IP's.  Your DHCP server might only assign a portion of it (that you can increase), or you have really ran out of IP's, and need a new subnet.  To increase the scope is the easiest:  Configure the server, and you're done.  A new subnet is a bit more involved.  Usually, you would add a VLAN, and assign a second scope on the DHCP server.  But the exact detail of how, will depend on your set-up and what you want to acheive.

    If I assume that your switches support multiple VLANs, and the router is connected with one Ethernet link to a switch, the steps would be:
     - Create a new VLAN on the switches (ensure that trunking is set-up on all network links).
     - Create a new sub-interface on the router.  Assign it an IP address out of the next subnet you want to use.
     - Create a new scope on your DHCP server for the new subnet.
     - Configure the router to forward DHCP requests from the new VLAN to the DHCP Server.

    Let us know what you want to do, and how your network is set-up, and we'll be able to provide more guidance.

    Author Comment

    1) We ran out of IPs, and need a new IP range.
    2) The rest you lost me on. My question (assuming I setup a new vlan on the router, a new DHCP scope for the new network. Does that mean that each request from old network to new network would have to go through the router?
    LVL 29

    Expert Comment

    1. OK
    2. Yes

    Don't make subnet larger than the normal 254 Host networks you get with a 24bit mask.  Ethernet looses efficiency around 250-300 hosts,...rolling back the mask by 1 bit will jump it to 510 hosts which is way to big to keep good performance.
    LVL 29

    Expert Comment

    2A.  Before you say it, the router won't be a bottleneck.  What it would be doing is exactly what routers were designed and built to do.

    2B.  The only traffic that crosses it would be traffic in which the specified destination demands that it cross the router.  All other traffic would not.


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