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Subnets

Discussion in 'Routing & Switching' started by Jon Christie, Jan 2, 2014.

  1. Jon Christie

    Jon Christie Bit Poster

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    Hi everyone,

    Could anyone please help me get my head around subnetting?

    I'm ok with binary and decimals but when it comes to the slash and number on the end ie /24, I get confused!


    Thanks in advance

    Jon
     
  2. Trogdor

    Trogdor Kilobyte Poster Gold Member

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    If you are comfortable with the concepts of the IP address octets and the binary to decimal conversion process, understanding CIDR (slash) notation is fairly simple. The number after the slash is the number of bits which make up the network address, i.e. for a /24 address, the first 24 bits of the IP address are the network address. In binary, this is 11111111.11111111.11111111.00000000. Converted to dotted decimal, this would make for a 255.255.255.0 subnet mask.
     
    Certifications: A+, Network+, Server+, PDI+, MCDST, HP APS Server, HP APS Desktop / Laptop
    WIP: ITIL, CCNA, MCSA, and BSc
  3. Jon Christie

    Jon Christie Bit Poster

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    Thank you, could you also assist me with variable length sub netting ?

    I appreciate the time you're taking to help me so thanks again :)
     
  4. BraderzTheDog

    BraderzTheDog Kilobyte Poster

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    Afternoon Jon,

    VLSM is just a flavour of subnetting you have already learned. I found it difficult to understand this when I was studying my Cisco quals, so I'll try explain it in the way that made the penny drop for me.

    As an example: You have you client network range of 192.168.1.0/24 and maybe you have another department say marketing that have joined the company. You need to give these machines IP addresses so they can access networked resources but all your other addresses are used up.

    The number of client PC's that use the 192.168.1.0/24 network is approximately 100 at peak time. This means you need to guarantee 100 addresses in that range are available for client PC's, however you will notice a /24 allows 254 hosts on that network. This means you have a gross waste of 154 addresses. This is where VLSM comes in.

    First of all you need to break the /24 into a smaller segment, but ensure you have enough addresses for the existing client machines. The first logical step down would be a /25 thats 255.255.255.128. This now gives you 126 usable addresses for the broken up subnet (now 192.168.1.0/25).

    So you may be wondering what happens to the other half of the subnet? Well you can mash that up according to your requirements.

    Sitrep: Currently the client LAN is now a /25 (192.168.1.0 255.255.255.128) with 126 usable addresses (192.168.1.1 - 127).

    Lets talk about marketing, this new department has 24 new machines and we need to give them addresses from the 192.168.1.X scope but they need to be on a different network for security reasons. We can't use 192.168.2.X or any other range because these are used somewhere else in our network. This is where we can now use the other half of our subnet we just broke up.

    Let try to preserve as many addresses as we can from the back half of the 192.168.1.X scope. We fist need to find something close to our requirement for addresses, in subnetting terms this will be /27 or 255.255.255.224. Why this partuicular subnet? Well the answer is look at how many useable hosts a /27 gives you.

    /24 = 254 - /25 = 128 - /26 = 64 - /27 = 32 - /28 = 16 Stop.

    A /28 isn't enough it will only provide 16 bits or 14 usable addresses for our marketing subnet. So the next bigger net will be /27. That provides 32 bits and 30 usable hosts.

    Therefore you allocate the following subnet for marketing: 192.168.1.128 /27

    Both ranges:
    CLIENT LAN: 192.168.1.0/25 (giving you 126 usable addresses)
    MARKETING: 192.168.1.128/27 (Giving you 30 usable addresses)

    A side note, you still haven't used the full 192.168.1.X scope. You can save this for later use / requirements.

    And thats it, you've variably subnetted.

    VLSM- VLSM Explained

    Regards,
    Brad.
     
    Certifications: CCNA R&S, CCNA-SEC, CCSA, JNCIA FWV, MCITP, MCTS, MTA, A+
    Jon Christie likes this.
  5. Jon Christie

    Jon Christie Bit Poster

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    Thankyou, will try to get my head round it :)
     
  6. bbel121

    bbel121 Bit Poster

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    The link that BraderzDog supplied should help. Here is also one that will generate random subnetting questions. Subnetting Practice Questions « Free CCNA Study Guide Free CCNA Study Guide Most people just need a bit of practice....more than the 5 or so questions you will get in the CCNA study guide that you purchase. Sit there for an hour and do a hundred or so of the how many valid hosts, what is the variable subnet mask, what is the first valid host, last valid host, etc questions and you will have it burned into your brain and be set for subnetting on the exam. :)
     
    Jon Christie likes this.
  7. vafangool

    vafangool Bit Poster

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    I have an excellent chart that is so simple to build to help with subnetting. I got it in school. .
     
  8. Jon Christie

    Jon Christie Bit Poster

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    I've got my head around it now buddy but a chart for reference would be great :)
     
  9. vafangool

    vafangool Bit Poster

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    Ok. If you want I can email it or try to post it on here
     
    Jon Christie likes this.
  10. zxspectrum

    zxspectrum Gigabyte Poster Premium Member

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    Very informative post this.

    Vafangool, i dare say the mods may let you post it on here, if not id like a copy as well please.

    Ed
     
    Certifications: BSc computing and information systems
    WIP: 70-680
  11. Jon Christie

    Jon Christie Bit Poster

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    Please post it here :)
     
  12. vafangool

    vafangool Bit Poster

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    OK I'll do it when I get home from school today. When you first look at it it's intimidating. But as long as you understand the power of 2 for binary it's simple. All you have to remember is 2 numbers and you can build it. I had learned subnetting from the ipv4 subnetting cbt nuggets video and I thought that was easier and I argued with the teacher saying it was easier. And he said wait until you get into ccna you will see. And he was right. You don't have to do any math. Everything is there. Your block size. How many subnets , how many hosts per subnet , the ciphr. It's very easy
     
  13. vafangool

    vafangool Bit Poster

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    Capture.JPG
    all you have to remember is a /24 cypher = 256 and you build down from there to start. you add one to the cypher and cut the block size in half... and you also know that a /24 bit subnet mask is 255.255.255.0 and that is one subnet....you keep adding one to the cypher and cutting the block size in half until you get to 30.. and you start from the 24 bit subnet mask at one subnet and keep doubling it for the number of networks.. and as far as knowing the subnet mask you just keep adding your binary weights...you know for a 25 bit cypher its one bit on in the 4th octet so that is 128 , a 26 bit cypher is 192 which is 128 plus 64, 2 bits on... a 27 bit cypher is 3 bits on 224..all you do is keep adding the binary weights for each column for the subnet mask... and keep doubling the amount of subnets, cutting the block size in half and adding one to the cipher.. So you have from 24 down and you want to go lower than 24..I go to the cipher colum and i start building from there...i take the 24 and keep writing the column up to 16, than on the block size colum you keep doubling till you get up to the 16 column..than for your subnet column you know that a 16 bit subnet mask is 255.255.0.0 all you do is keep adding your binary weights like you did for 24 down but in the 3rd colum, and you also know that 16 bit subnet mask also creates 1 subnet. so in that column you keep doubling the amount of subnets until you get all the way down to the 30 cipher...so when you build from 16 down its also the same thing if you want to build from 8 down..and any question you can get asked the answer is right there in that table... say you get a question where you need to know the block size of a /18 now you now the block size isnt 16,384, so there is 2 ways to figure it out... you can either look at the subnet mask of a /18 and you see its 255.255.192.0 ..you can either minus the 192 from 256 and that is your block size which = 64 , but instead of doing the math you can just look at the other cipher at the bottom with the mask that has 192 in the last octet and the block size is the same 64, ...

    I hope i explained this easy ...its very easy to build and any question that you get asked the answer is right there... you have your cipher, your block size your subnet mask and also the amount of subnets that it makes... any questions guys just let me know....

    - - - Updated - - -

    there it is...any questions let me know..i hope this helps..it really is once you understand how it works...
     
  14. Jon Christie

    Jon Christie Bit Poster

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    Thankyou vafangool, it'll be a handy reference on a foggy head day :)
     
  15. vafangool

    vafangool Bit Poster

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    Do you see how easy it is to write out?
     
  16. Jon Christie

    Jon Christie Bit Poster

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    Sorry for the late reply, but thanks to all that have helped me :)
     

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