What is the best way to change my network ip and subnet?

Posted on 2009-02-20
Last Modified: 2012-05-06
I am running out of ips on my current network 192.1.4.x and am having trouble determining the best way to go about it.  

                                                                         / cisco 2800 series vpn router--\
wan -----cisco 2800 series ip router---switch                                                   network (win 2k3 server handling dhcp)
                                                                         \ pix 506e -----------------------------/

That is an awful drawing of my setup. I have static ips setup for part of the 192.1.4.x and dhcp for the rest.  I would like to move the easiest way possible to at at least 255 dhcp address and 255 static ip addresses.  Optimally I would like around 510 for each so that there is significant growth room.

My first thought was to supernet a 192.168.0 address with a subnet.  I would then use 192.168.2 and 192.168.3 for dhcp and 192.168.0 and 192.168.1 for static ips.  My concern with this is that my dhcp server/vpn router/pix is not letting the dhcp requests get through properly to my dhcp server so it never hands out any leases when I deactivate the 192.1.4 dhcp scope.  I did connect another network connection to the dhcp server and give it a static ip on the 192.168.0 network with subnet mask of but left the original connection to my 192.1.4 network on as well with a different subnet mask.  This could be the problem or it could be impossible I do not know.
My other concern with this setup is too much broadcast traffic.  Is having 1020 addresses too much on the same subnet?

Could someone please help me in determining if this is possible or if it would be a very bad idea to have all the addresses on the same subnet or suggest something else I could do.
Question by:uescjp
    LVL 8

    Assisted Solution

    you can do the 192.168.0.x supernet for dhcp and for static ip. However I would still start of with the class C of addresses of for dhcp and for static ips initially and then just change the subnet mask to then, then, etc on the gateway interfaces as i need to expand. Just incase there is something else you want to do down the road.

    Author Comment

    API NOC: I understand your suggestion but should I have that big of a supernet with no network segmentation?

    Do I have to remove the 192.1.4 scope and static ip from my DHCP server before I can have my DHCP server recognize the new DHCP scope even if I have a different NIC in the server on the new network?

    Will I take a great performance hit on my network having the relatively large unsegmented supernet due to broadcast collisions?

    I am possibly exploring a layer 3 switch to try to segment my network but is it really necessary?
    LVL 8

    Accepted Solution

    Just change your mask to
    This will give you -

    It's not as big as the other recommended supernet, and you only need to change the subnet mask in the DHCP scope.

    Alternative if you would like a larger scope, you could just use the scope with your existing range to give you

    As for the broadcast traffic it depends on whats going on on your network as to if it will be a problem or not. I recommend running wireshark and seeing how much broadcast traffic is running around at the moment and expand that rate to allow for your network expansion.
    LVL 8

    Expert Comment

    Broadcast traffic was really a problem back when switches and devices were 10M Half-Duplex. With 10/100/1000MFD switches you should not see that much of a problem if any. Like the above comment you can you wireshark or any kind of network analyzer that will report broadcast statistics, then adjust accordingly.
    LVL 8

    Assisted Solution

    You seam to be slightly mixed up between the concepts of broadcast domains and collision domains. Broadcasts do not collide; they simply get forwarded to all of your hosts. In a modern network using switches (not hubs) you should not be experiencing any collisions since the switches segment your collision domains down to a single wire. This is why connections can operate at full duplex.

    The size of your broadcast domain is not nearly as important as the number of broadcasts. It would not matter if you made the the subnet a if you do not generate a significant number of broadcasts. Your broadcasts generally will grow in proportion to the number of hosts in the broadcast domain. The two most prominent type of broadcast I see on my network are DHCP requests and ARPs. If you have a significant number of hosts staticly coded then you should factor in that reduction in broadcasts.

    The normal high end limit you would want to see for amount of broadcasts is roughly 5% of total traffic. If you are seeing broadcast numbers higher than that than you defiantly want to segment your broadcast domains.

    Author Closing Comment

    Thanks.  I never got an answer to what I was exactly trying to do but I like the subnetting 192.1.4 instead of trying to move everything to 192.168.  Also thanks for the additional information.

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