How to setup CIDR in a bussiness setting. (Help)

grave_diggergrave_digger Senior MemberMember Posts: 127
So I been trying to learn CIDR with my Cram Exam book and looked up some scenarios to work out and I found one.

So this company has six departments with certain amounts of computers to be in each depart. I have ben given to be used as the CIDR for this network. Each department has various amount of computers and I have to know what CIDR I would give that section of computers for each department.

Accounting has 53 computers
Sales 15
HR 8
marketing 33
customer service 24

How do I do the math to figure out which CIDR address I give each department that will appropriately give the right amount of IPs for each amount of computers given?
1776 is the answer to 1984!


  • ebohlmanebohlman Junior Member Member Posts: 26 ■■■□□□□□□□
    Hint: you need to start with the biggest subnets first, so first you do accounting, then you do marketing, then you do CS, then sales, and finally HR.

    Just looking at accounting, remember that your subnet size has to be a power of 2, and you lose two addresses (your network number at the beginning and your broadcast at the end). The smallest power of two that's greater than or equal to 53 is 64, so you'll need to allocate a block of 64 addresses to accounting. It's a good rule to start at the beginning of your address space, so that block should be through (the usable range will be through

    That means that accounting will be, corresponding to a mask of, and the next available addresses will start at Now do the same thing for the rest of the departments, keeping in mind that the address ranges will usually be of different sizes.
  • SimridSimrid Senior Member Member Posts: 327
    The CIDR is a represnation of the active bits in each segment on a IPv4 address. This has to be done in binary, let me explain.

    /24 address allows for 254 usable hosts. This can be written like so 11111111.1111111.11111111.00000000

    If you notice, there are 24 1's in this address. Each 1 accounts for a number (reading left to right) 2,4,8,16,32,64,128,256. Therefore, add the 1's together and that is the value in that octet. CIDR is the quick hand way of displaying this binary, or a the subnet mask

    Hope this helps.
    Network Engineer | London, UK | Currently working on: CCIE Routing & Switching
  • volfkhatvolfkhat Senior Member Member Posts: 1,022 ■■■■■■■■□□
    Ebohlman Nailed it.

    i can't really add anythig more.

    The question you posted is asking about the concept of VLSM (Variable length subnet masks).
    If you don't fully grasp it... then search for alternative materials (until you find something that clicks).

    Personally, i praise the Magic # method.

    Try Danscourses:

    He is quite talented.
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