VLSM is doing my head in!!!!!!

Vex10Vex10 Member Posts: 30 ■■□□□□□□□□
Hey guys,

Does VLSM show up in the exam, I cant seem to understand it.

Any help would be greatly appreciated.



  • dynamikdynamik Banned Posts: 12,314 ■■■■■■■■□□
    Multiple study resources that I'm using explicitly says it does not, but I think I've come across a VLSM question or two in Transcender.

    Be sure to convert to binary, at least while you're getting started. Going about it that way makes it pretty straight forward.
  • Vex10Vex10 Member Posts: 30 ■■□□□□□□□□
    Cool thanks for the heads up.
  • mrx9000mrx9000 Member Posts: 37 ■■□□□□□□□□
    I agree, I always write it down - 11111111.11111111.11111111.11111111
  • ClaymooreClaymoore Member Posts: 1,637
    By VLSM do you mean the /?? notation (commonly refered to as CIDR notation) or do you mean subnets other than the traditional classful boundaries such as Or do you really mean using different classless subnets within the same classful boundary, such as a mixture of /28 and /27 subnets in a classful network? I don't specifically remember the test using CIDR notation, but you can certainly expect a network to be subnetted in all sorts of different ways. You need to understand subnetting, but try not to get too wrapped up in the network IDs and masks listed in the questions if you only really need to know where to install a DHCP relay agent.

    Here is a VLSM example from a Cisco Technote:

    VLSM Example
    Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:

    netA: must support 14 hosts
    netB: must support 28 hosts
    netC: must support 2 hosts
    netD: must support 7 hosts
    netE: must support 28 host

    Determine what mask allows the required number of hosts.

    netA: requires a /28 ( mask to support 14 hosts
    netB: requires a /27 ( mask to support 28 hosts
    netC: requires a /30 ( mask to support 2 hosts
    netD*: requires a /28 ( mask to support 7 hosts
    netE: requires a /27 ( mask to support 28 hosts

    * a /29 ( would only allow 6 usable host addresses therefore netD requires a /28 mask.

    The easiest way to assign the subnets is to assign the largest first. For example, you can assign in this manner:

    netB: host address range 1 to 30
    netE: host address range 33 to 62
    netA: host address range 65 to 78
    netD: host address range 81 to 94
    netC: host address range 97 to 98

    Also, learntosubnet.com is a useful site for, well, learning to subnet.
  • Vex10Vex10 Member Posts: 30 ■■□□□□□□□□
    Thank you Claymoore you have cleared that up very good.

    The cisco technotes is good to, thank you again.

  • mattd75mattd75 Member Posts: 22 ■□□□□□□□□□
    How do you have a CCNA and not understand VLSM? I assumed that would be a huge part of passing any Cisco exam?
  • Vex10Vex10 Member Posts: 30 ■■□□□□□□□□
    It was but that was ages ago and I have not used it in along time, with me my head is like a bucket with holes. If I dont use it goes. Even the it took me ages to understand it through reading it over and over.
  • Dracula28Dracula28 Member Posts: 232
    I had a VLSM related question on the exam, so it can show up on the exam, even if the training kit says that its highly unlikely.
    Current certs: MCP (210) MCSA (270, 290, 291 and 680) MCTS (680, 640)
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