How to do subnetting in Cisco ?

Posted on 2011-10-08
Last Modified: 2012-12-16
Hello Expert , I am doing  A ccna course , and really intrest me , now we got to the subnetting chapter and I find it hard one , I have got a book , and there are many ways finding soluations to ne problem , my question is is there a simple way where u can answer the most common question like , what network certain IP address is on? what is the subnet mask , the broadcast address and the range , pls I gt exam on monday and I want to do all I can to understand and pass the exam , what confuse me the most is the no of bits that r needed to design a network , or when they say for example you got and ur requirement is at least 500 hosts per network however as many subnets as possible without exceeding 1000 subnets , I need explaining pls and not throw a website and I need to deal with it , I need to understand more than anything else . thks alot
Question by:hnael
    LVL 6

    Expert Comment

    If you have already registered with Cisco, the learning center helped me to understand this topic a whole lot better. Check out

    Author Comment

    Sorry , I could not view the vedios because I gt a probelm with registration, since I was registered in the past and it seems I gt 2 registration and I tried 20 times to register with no luck , can you send me a direct link pls without going throw registration, thks
    LVL 12

    Accepted Solution

    I'll try to explain as simple as I can.
    All about subnetting is related to understanding of binary representation of adresses and masks and calculating with with 2^x.
    So you have Mask for this network is or /8 (255 is equal to eight 1 in binary representation of subnet mask).
    To have 500 hosts in one subnet, you have to find smallest "x" in 2^x > 500
    2^1 = 2
    2^4 = 4
    2^3 = 8
    2^9 = 512

    So subnet you need should have last 9 zeroes: or /23 (maximum nuber of 1's is 32, so 32-9=23).
    That is for hosts.

    To calculate number of subnets which contains 1000 /23 networks, do the following:
    - first find suitable supernet mask:
    2^x >= 1000
    By same system as above x=10, so you need to add 10 zeros to existing 9, to have 19 in total.
    Converting it to mask: or /13.

    To calculate final number of networks with 1000 subnets consisted of 500 hosts each, you have to find how many /13 networks are in /8.

    I hope this clears things a bit, and I hope I understood your question well. If not please post original question from your test.



    Author Comment

    Any more easiar way to find the no of bits needed for subnetting ?
    LVL 12

    Expert Comment

    One thing to remember: All 1's and 0's in subnetting must be continuous.

    Easiest way is to memorize all 2^x, where x is 1 to 10. In converting that to subnets, x defines number of 0's at the end of subnet mask.

    In my previous post, whole process is described in more detail, but in essence it is just finding closest bigger 2^x (example: for 200 hosts, first bigger is 2^8=256).


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