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Thread: VLSM question

  1. #1
    Join Date
    Oct 2009
    Posts
    4

    Default VLSM question

    Hi!

    I have a question about VLSM design. Let's take the example from the Study Guide on page 138.

    So, we have following subnets:

    1) 192.168.10.0/27 (30 valid hosts, 25 used)
    2) 192.168.10.32/28 (14 valid hosts, 13 used)
    3) 192.168.10.48/28 (14 valid hosts, 10 used)
    4) 192.168.10.64/29 (6 valid hosts, 6 used)
    5) 192.168.10.72/30 (2 valid hosts, 2 used)

    My question: what should I do if I will need to expand the subnet 10.48 to 30 hosts after network is designed and functions? How to solve expanding issues with VLSM design?

    Thanks ahead.

  2. #2
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,422

    Default

    What you should do is think ahead, and plan for expansion. Then you can design your network addressing scheme. If you got into a situation that you needed to expand a subnet, but simply choosing a new mask would encroach on used address space, you would have to find another subnet completely, and re-address all devices in that subnet.

    For instance, lets say you have 3 subnets

    192.168.0.0/23
    192.168.2.0/25
    192.168.2.128/25

    You need to expand the second one to create enough addresses for 250 hosts. You can't just use a new mask, because the third subnet is already using half that address space. You can't use anything from 192.168.0.0-192.168.1.255, so your only option is to move to something like 192.168.3.0/24. All devices would then need to be re-addressed.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  3. #3
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    We get this at work all the time. People not planning ahead and needing more IP addresses and public ones at that.

    Nightmare.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  4. #4
    Join Date
    Oct 2009
    Posts
    4

    Default

    Hi!

    So, I understood that it is very hard task to expand a subnet with VLSM.

    Thank you very much for your replies.
    Last edited by zeronoel; 10-19-2009 at 06:30 AM.

  5. #5
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,422

    Default

    Only if you don't plan ahead. If you use the following scheme

    192.168.0.0/27
    192.168.0.64/28
    192.168.0.96/28
    192.168.0.128/29
    192.168.0.144/30

    Now you have a used subnet followed by a blank subnet of the same size, the next used subnet followed by another of the same size etc. Now if you need to expand on the second subnet, you can easily change the subnet mask to /27 and add more hosts. All the current hosts will still work because you haven't had to completely change the address. All they need is the new subnet mask info.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

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