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Help With CCNA Question - VLSM/Subnets

Discussion in 'General Cisco Certifications' started by Anthonyuk, Mar 3, 2008.

  1. Anthonyuk

    Anthonyuk New Member


    I have been reading every text book I have and googling everything I can think of for the last two days but I can not for the life of me find any useful info on how to tackle this, likely simple to those in the know, question I have for my CCNA course.

    I don't want someone to give me the answer as there is no point it this, but I would like some help on material giving relevant examples on figuring it out.

    Ok the question is in three parts:.

    Given the class A network create a subnet mask for the """ subnets.

    (I have put """ as I don't want help in answering this specific number of subnets, but its a large number).

    Then it asks identify the """th (also a very large number).

    Then VLSM the """th (also a large number)

    From reading Lammles book all the examples I can find start with the Subnet mask and work from there forward.

    I can not find anything in the book on starting from the network to obtain a subnet mask for only a certain subnet amount.

    I have read the default subnet for is and worked this out into 11111111.0000000.00000000.00000000 into binary.

    But where to go now?

    Obviously it will be easy to people that know this but like mathematics, for my mindset at least, it is impossible to see how something is done without seeing similar examples before tackling your own.

    Thanks for any help anyone is able to provide.
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    Hi and welcome to CF!

    The standard answer often given here is to look at this site for a tutorial on subnetting.

    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. JonGlory

    JonGlory Byte Poster

    I dont know if i picked up the question right but to get the number of bits you need to create x amount of subnets, just think of it like this.

    If i borrow 1 bit i get 2 subnets
    if i borrow 2 bits i get 4 subnets
    if i borrow 3 bits i get 8 subnets
    if i borrow 4 bits i get 16 subnets

    see the trend?

    add 1 to the bits and times the subnets by 2
  4. CheeseOnToast

    CheeseOnToast Nibble Poster

    well, the guys above are heading in the right direction

    But with VLSM you are actually taking the IP you are given and breaking it down in hierachal fashion to use your address resources effectively without waste

    and yes you are assigning a subnet-mask which is appropriate to the situation

    To be honest, I have only learned it myself the past week or so, and I don't think it is the type of thing that can be taught on the forum, I would google it and google some VLSM practise papers and basically just keep going!

    here is a small eg -

    Take Class C address

    If you subnetted this using a /26 mask, you would be left with 62 useable hosts per subnet (6^2-2).

    So lets say you use subnet 0 for your first subnet, and you use this on a P2P serial link between two routers, you would use 2 addresses, wasting the other 60 addresses!

    This is the point of VLSM.

    You could take subnet 0, use the 62 hosts for say your PC's in one building. Take the following subnet, which in this case, would be, and then further subnet it, e.g. using a /27 mask if your other building only needed 30 useable hosts... then take the further subnet e.g. and subnet it with a /30 subnet mask, yielding 2 useable hosts for your P2P link...

    Do you see the kind of point I'm trying to make>?

    Maybe my eg is crap - but you need to read up and do some practise ones to be honest and come back with any specific q's

    Sorry to anyone if I've made any mistakes in that above!! lol
    Certifications: ITIL Foundation
  5. sunn

    sunn Gigabyte Poster

    I can’t tell if this is a subnetting question or a summarization question.

    Subnetting :
    These are aspects pertaining to subnetting:
    (i) Number of Subnets? // (2ⁿ) -where n is number of 1’s (ip subnet zero is used)
    (ii) The specific subnets? // (256 – subnet mask) = base number (starting point)
    (iii) Broadcast for each subnet? // The address preceding next subnet
    (iv) Number of hosts per subnet? // (2t - 2) where t is number of 0’s
    (v) The Hosts in the subnet? // Hosts are the addresses between the subnets (dropping the network and broadcast address or all 0’s and all 1’s respectively)

    Example: (or '/28')

    Number of subnets = (2^4) = 16
    What subnets = (256 – 240) = 16. Subnets are 0 | 16 | 32 | 48 | 64 | …
    Number of hosts / subnet = ((2^4)-2) = 14

    Network: Network:
    Host Range: 206.235.255.(1-14) Host Range: 206.235.255.(17-31)
    Broadcast: Broadcast:
  6. Yido

    Yido New Member

    In the question it should state how many networks are needed or how many hosts are required.
    This is usually done with a prefix or slash notation (/) to define the subnet mask.
    If they want you to use the default subnet mask then this is unsubnetted and in effect is just 1 large network.

    Dont think youve put the whole question down unless im getting the wrong end of the stick.
    Certifications: Bsc (Hons) Computer Science
  7. sunn

    sunn Gigabyte Poster

    Here's the summarization example:
    i) This is a crude example, but it gets the point accross. Crude becuase the summarization is too broad.

    Take the following network addresses:

    1) Convert the addresses to binary format and align them in a list.

    --Address--------1st Octet--2nd Octet--3rd Octet--4th Octet 10101000 01100010 00000000 10101000 01100011 00000000 10101000 01100100 00000000 10101000 01100101 00000000 10101000 01100110 00000000 10101000 01100111 00000000 10101000 01101000 00000000 10101000 01101001 00000000

    2) Find where the common pattern of digits ends. (i.e. 11000000 10101000 0110xxx xxxxxxxx) x's are uncommon bits
    3) Count the number of common bits. The summary route number is the 1st IP address in the block, followed by the mask (or CIDR notation).

    Summarized route is 11000000 10101000 01100000 00000000
  8. CheeseOnToast

    CheeseOnToast Nibble Poster

    Funny, I've never been shown before that you can do 256 - Subnet Mask to get the starting point of the subnet ranges

    Bloody useful one to know!

    I always end up quickly jotting the binary down lol
    Certifications: ITIL Foundation
  9. sunn

    sunn Gigabyte Poster

    Doing it in binary will obviously work, but you'll probably run out of time during an exam. Use the 256 (magic number) method and you'll save precious time.
  10. CheeseOnToast

    CheeseOnToast Nibble Poster

    I repped you kind sir :p
    Certifications: ITIL Foundation
  11. r.h.lee

    r.h.lee Gigabyte Poster


    Check out the following link.

    1. IP Addressing and Subnetting for New Users - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml
    Certifications: MCSE, MCP+I, MCP, CCNA, A+
  12. purplejade

    purplejade Nibble Poster

    Normally in CCNA exams you will have atleast 2 to 3 questions on VLSM & Summarisation, these will be of pick and drop sort questions where you have a network diagram giving you the no of hosts in each network, and you have to allocate the address from the given choices. the address will all have cidr notations, and as long as you know your blocksizes and work out quickly the no of subnets and hosts for each given ip address it should be fairly simple and easy..you can find practice examples in cd accompanying the ccna press books...
    Certifications: CCNA N+
    WIP: A+

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