Subnet full

Posted on 2007-10-03
Last Modified: 2010-04-10
Hi There,

I have a subnet at a site that is just about full. Its a C class (Eg

I have servers, workstations and printers in this subnet. I have thought of splitting servers into their own subnet (via a VLAN) but am worried about the performance implications of this. If i have to do inter VLAN routing will this reduce the bandwidth available (and increase latency) for the workstations to access servers?

The other option is to stretch out the subnet mask for the existing network (Eg. but im already using the above network at another site (Eg

Any thoughts?

Question by:thegewse

    Expert Comment

    Vlan's will be the easiest solution if you have a good layer 3 switch to do the routing for you.  Bandwidth latency will be dependant on the hardware.  I have not experienced latency on HP and Cisco, 3Com.
    LVL 8

    Assisted Solution

    A good practice is to change the subnet mask:
    if you use:
    your LAN range will be from to (512 IP avaliable)
    I would configure DCHP to range to
    and let 192.168.0.x be static ip range for printers, routers,switches and servers

    The only problem is that you have a 192.168.1.x LAN, so maybe you sould think about changing the IP rank to something like

    I think you have to worry about future growing, and changing mask is the easyest way: if you then reach more than 512 machines, you can change the mask to /22 and get 1024 IPs (
    LVL 7

    Expert Comment

    VLAN routing is another task your switches will have to dill with so, in theory you will pay performance penalty.
    However, today's network infrastructure can handle these kind of tasks with latency of a few nano-seconds.

    As for the other option, it is recommanded under 2 conditions:
    1. You don't own a 3rd layer switch (why buying expensive HW if you can provide solution without costs?)
    2. Your other site has no direct connection to the first site (if it has, you'll have to change the other sit's IP subnet to say

    Author Comment

    Hi All,

    Thanks for comments.

    Regarding the VLAN option, If i have to create a VLAN subinterface on the LAN interface of one of my routers to route between VLANs. Wont all the traffic from the Server VLAN to the Workstation VLAN have to cross this interface?

    If this is a 100Mbit interface will the maximum bandwidth available for all workstations to reach servers be 100Mbit? This would be a significant drop compared to the 1Gbit uplinks i have joining switches currently
    LVL 7

    Accepted Solution

    Today's switches are smart enaugh to direct rout your traffic if the detination IP is directly connected to the switch. That although the VLAN configuration.
    In case the destination IP isn't directly connected to the switch you are right, all traffic will pass through the devided interface.
    If you already have the HW, you can try it for a couple of days.
    In any case, increasing your subnet by definning a 24 bits mask is the easyest way to deal with the problem.
    Just set your DHCP to exclude and make the right routing rules.

    Why not change the whole IP network to it will solve your problem with no duplications at all?

    Author Comment


    I think the final solution will be to increase my subnet size, but this will not happen until i have a chance to reconfigure all infrastructure.

    I might make a "printer" VLAN in the mean time to give me some breathing space..

    Oriziv: I have a private network that uses the range and i dont want to confuse internal LAN ranges with private network ranges..

    Thanks for the good advice. Points split between Mikk and oriziv

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