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Subnetting (Big breath in and sigh)

Discussion in 'General Cisco Certifications' started by Andy_M, Jul 17, 2010.

  1. Andy_M

    Andy_M Bit Poster

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    Ok, won't moan to much to make another thread about how hard Subnetting is. But for real I am struggling to get to grips with it, can only seem to to calculate the subnet without outstanding why I am doing it, what it relates to in the real world, and how to do it fast when it takes me about 5 minutes for each one (using CBT Nugget video steps)

    I mean

    195.5.20.0/30
    255.255.255.252

    why would this be broken into 50 networks with two useable IP's? am I missing the point?:eek:
     
    Last edited: Jul 17, 2010
    WIP: CCNA
  2. SimonD

    SimonD Terabyte Poster Moderator

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    Have a look at this thread and see if it makes more sense after reading.
     
    Certifications: CNA | CNE | CCNA | MCP | MCP+I | MCSE NT4 | MCSA 2003 | Security+ | MCSA:S 2003 | MCSE:S 2003 | MCTS:SCCM 2007 | MCTS:Win 7 | MCITP:EDA7 | MCITP:SA | MCITP:EA | MCTS:Hyper-V | VCP 4 | ITIL v3 Foundation | VCP 5 DCV | VCP 5 Cloud | VCP6 NV | VCP6 DCV | VCAP 5.5 DCA
    WIP: VCP6-CMA, VCAP-DCD and Linux + (and possibly VCIX-NV).
  3. DC Pr0Mo

    DC Pr0Mo Kilobyte Poster

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    One reason for using a network with only two ips is for a router to router connection. my advice with subnetting is not to try and understand why you would do it a certain way, but more understand that you can, if that makes sense.
     
    Certifications: MCDST | BSc Network Computing
    WIP: 70-291 | 70-293 | 70-294 | 70-297
  4. Andy_M

    Andy_M Bit Poster

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    Not really, I can work out the Subnets but not sure how this is suppose to be done fast for exams, and also if those charts work?
     
    WIP: CCNA
  5. Sparky
    Highly Decorated Member Award

    Sparky Zettabyte Poster Moderator

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    Well for branch office setups an ISP can give you two published IPs, one for the router and one for the firewall.
     
    Certifications: MSc MCSE MCSA:M MCSA:S MCITP:EA MCTS(x5) Security+ Network+ A+
    WIP: Exchange 2007\2010
  6. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Because there's no reason to waste the whole 195.5.20.0/24 range on connecting two devices in a point-to-point configuration. Like the previous posters mentioned, a /30 mask is typically used for a link between two routers.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  7. gosh1976

    gosh1976 Kilobyte Poster

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  8. Andy_M

    Andy_M Bit Poster

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    None of it seems to help, I am now even lost calculating the subnet as I have seen four ways to subnet, the teacher says we are covering it on Wednesday in more detail. I am getting so f*@ked off with it, it isn't sinking in for me at all.

    youtube has waste of time videos that isn't helping, my maths isn't even that bad, binary is easy. but I don't get the whole host/network concept or subnetting.

    I feel like I'm 10 about to have a hissy fit.
     
    WIP: CCNA
  9. DC Pr0Mo

    DC Pr0Mo Kilobyte Poster

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    It can be hard concept to grasph. Try to think of a IP Address like a phone number (01292 553322) which has an area code (01292) for the local telephone exchange and the rest is your local telephone number (553322), the only point of a subnet mask is to point out what part is the area code (Network) and what part is the local number (Host address).

    Not a perfect example but may help.
     
    Certifications: MCDST | BSc Network Computing
    WIP: 70-291 | 70-293 | 70-294 | 70-297
  10. greenbrucelee
    Highly Decorated Member Award

    greenbrucelee Zettabyte Poster

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  11. Danmurph

    Danmurph Byte Poster

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  12. StoneTZ

    StoneTZ Bit Poster

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    I know how frustrating it can be, it was for me until all the sudden it clicked ..finally.
    I use a method similar to www.subnetting-secrets.com I haven't clicked on all the links in this post so maybe there is something similar. For me the chart makes it 100x easier and faster. I went from about 2 or 3 minutes and getting burned out to doing them in 5 to 30 seconds and it being easy.

    In regards to why you're doing it and what it all means... I think I understand it well enough but it's so hard for me to explain. Basically what it comes down to though is you're evenly dividing the network. Say the network had went from 1 to 100 (they don't it's 0 to 255 4 times but to explain it easy)

    And I said to you "you can divide this 100 any way you like" and then you said "hmm I'll divide it by 20's" you would have:

    1-20
    21- 40
    41- 60
    61 - 80
    81 - 100

    now you divided 100 by 20 so you got 5 groups of 20. Thus you would have 5 networks and 20 hosts each.
    Heres the catch, your first number of each is your net number and the last is your broadcast address so you would really have 5 nets and 18 hosts each. net= 1, broadcast = 20 for the first one. 21 and 40 etc

    Obviously this is a simplified not how it really is answer. numbers range from 0.0.0.0 to 255.255.255.255 and increase by increments of 2,4, 8, 16, 32, 64 or 128 and other fun stuff that I left out. I just wanted to see if I could help on what is happening when you break it down to networks and hosts. Also you can see with the network and broadcast addresses why you lose out as you create more networks.

    If this is confusing or someone who really knows what they're doing thinks this is bad advice please ignore it =) If it's horrible horrible stuff I'll edit it out =) I'm going on little sleep and not sure if I'm the one who should try to teach...
     
    Certifications: A+
    WIP: Net+, CCNA
  13. Andy_M

    Andy_M Bit Poster

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    All thank you for your replies, It has finally clicked to why we do subnetting and the reason behind doing this. The confusion was that I thought the public IP would also be the same as the private network (Because this is true for where I work *University*)

    I now understand you have a few Public Addresses and can use almost any Private Addresses for the networks, i also understand how to subnet but cannot do this in the time required and takes me about 5 minutes.

    I am going to study, study and study more with practise in between studying for subnetting as I feel this is the hardest part of the CCNA (ICND1).

    I thought I could book my ICND1 exam next thursday but know this is a mistake as I need to be able to subnet as second nature.

    I will keep you all updated and also try and ask questions as and when I get stuck, and hopefully can help people on here in the future once I obtain my first part of the CCNA,

    I have alot of respect for any CCENT or higher member as this really isn't a walk in the park.
     
    WIP: CCNA
  14. ChrisH1979

    ChrisH1979 Byte Poster

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    Keep at it and it will "click" sooner or later. Took me a while as I really don't have a maths brain. Keep having a go but take breaks and it will come. I find using the charts helped the most and using the power of to work out hosts.
     
    Last edited: Jul 24, 2010
    Certifications: MCITP:SA, MCSA, MCTS:Win 7, Application Infrastructure
    WIP: MCITP:EA
  15. StoneTZ

    StoneTZ Bit Poster

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    Man the charts helped me so much. I would highly encourage you use one unless you can do it in your head faster and accurately.
     
    Certifications: A+
    WIP: Net+, CCNA
  16. morph

    morph Byte Poster

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    it took me ages to get it - i tried all sorts of techniques - used this website everday www.subnettingquestions.com - use charts maybe to help u understand it, but if u can get to the point of doing it in your head - thats the key. Believe me once you've got it it seems easy, and thats half the frustration i think, keep pluggin away dude - you'll get it!
     
    Certifications: Network +, ITIL Foundation, CCENT, CCNA
    WIP: server/ccna security
  17. StoneTZ

    StoneTZ Bit Poster

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    When you say do it in your head do you mean looking at the chart? Or do you mean you just stare at it and say "the first host is 192.168.0.64!!!"
     
    Certifications: A+
    WIP: Net+, CCNA
  18. jonny7_2002

    jonny7_2002 Byte Poster

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    I tried my CCNA back in 2007 with the old exams and ended up leaving it because i got stuck on subnetting...... :-(

    I picked up ICND1 2 years later and got the CBT Nuggett videos which made it 'click' for me! Jeremy Cioara is a legend!

    the guys are right.... you will suddenly get it and then think "i was doing that already but didnt realise!!"
     
    Certifications: CCNA R&S, CCNP R&S, CCDA, CCNA Voice, CCNA Wireless & CCNA Security
    WIP: CCIE V5 (when its out)
  19. morph

    morph Byte Poster

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    Don’t know if this helps, but show you how I work it out :

    say I have 192.168.100.17
    and it’s been given a /27 mask - I know from doing it so much that a /27 = 255.255.255.224
    working in the 4th octet - 255 -224 = 32 that’s my increment, I now know the range is 192.168.100.0 - 192.168.100.31

    If I want to know how many networks - again I’m in the fourth octet - 255 divided by 32 = will give 8 networks.
    I would say learn how to do that first, in your head.

    Then look at working out masks and hosts and number of networks from there.

    For example I’ve got 192.168.100.0 - I need to give it a mask for say at least 10 networks so it’s going to be an initial mask of 255.255.255.somthing - if I give it a 128 - that only gives me 2 networks, as 128 and 128 make the 255. I need to go to 4 networks - half of 128 = 64 - still not enough I need to go through to 16 = which will make the mask 240 as I take the 16 of the 255
    so 192.168.100.0 255.255.255.240 gives me 16 networks with the following ranges.

    I’ve just read that back and probably haven’t explained myself very well, it’s one of those things keep doing it and doing it and it'll just click - I knows it’s really frustrating - keep at it though ;) people like it when u can do it in your head...well it wont impress chicks but u know :)

    ps- use www.subnettingquestions.com all the time!
     
    Certifications: Network +, ITIL Foundation, CCENT, CCNA
    WIP: server/ccna security
  20. gosh1976

    gosh1976 Kilobyte Poster

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