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Subnet Mask Cheat Sheet

Discussion in 'General Cisco Certifications' started by tripwire45, Nov 28, 2005.

  1. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    Thought some of you might need this.

    http://krow.net/dict/subnet.html
     
    Certifications: A+ and Network+
  2. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    Nice find Trip! :thumbleft
     
  3. Boycie
    Honorary Member

    Boycie Senior Beer Tester

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    thanks Trip :thumbleft
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  4. ffreeloader

    ffreeloader Terabyte Poster

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    This is an area where I think a cheat sheet does more harm than good for a newbie. A newbie needs to do the math to figure this out manually every time they do it so the concepts really stick in their heads.

    The training provider I studied with for a while handed out cheat sheets and the guys who always relied on the cheat sheets really struggled understanding subnetting. They also struggled on the tests because they had always relied on cheat sheets to do the work for them, and they couldn't take a cheat sheet into the exam room so they were lost when it came to figuring things out then.

    It is almost as quick to use binary math as it is to use a cheat sheet. The number of valid IP addresses in a /24 subnet is 2 to the 8th power -2. It's really easy to remember because /24 is a Class C address. That means it uses 8 bits and 2 to the 8th is 256. Subtract 2 and you have 254 valid addresses.

    A /26 subnet is 2 to the 10th power minus 2. That 1022.

    Say you are breaking up a Class C subnet. If you use a /30 subnet you have 2 to the second power minus 2 valid IP addresses per subnet. You also have 2 to 6th power subnets because 8 - 2 = 6. That's 64 subnets with 4 valid IP's per subnet. It's possible to break out any combination of subnets using this method, even if you're subnetting a subnet of a larger whole.

    If you're using RIP v1 for your routing protocol then you have 2 to the 6th power -2 subnets per Class C address space.

    This is very easy to do. You have 8 bits in a Class C address so whatever you use as your subnet mask you count that many bits to the left and make that your dividing point. Then the bits left to the left of that dividing point count toward the number of subnets and bits to the left of that dividing point count toward the number if IP addresses. This will work for subnetting any possible combination of bits in an block of network address.

    Never use a cheat sheet when you have something this simple to use for doing subnetting. You'll look like a genius doing this in your head....
     
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  5. Bluerinse
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    Bluerinse Exabyte Poster

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    Freddy I think you need a *right* in there somewhere :biggrin
     
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  6. Clyde

    Clyde Megabyte Poster

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    freddy is correct though...


    reliance on cheat sheets is a waste of time. If you know subnetting you won't need one, and if you need one, you don't know subnetting enough to pass the exams
     
    Certifications: A+, Network+, Security+, MCSA, MCSE
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  7. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    Whilst freddy is correct I just have to say that from my perspective, someone who found subnetting one of the hardest concepts I have ever tried to grasp, that it can sometimes be easier to understand how to work things out if you already know the correct answer.

    8)
     
  8. ffreeloader

    ffreeloader Terabyte Poster

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    I'll try to come up with a visual for my concept so you can actually see what I'm doing Simon. Once you do it a few times you'll be amazed at how easy it is. It's far easier to remember how to do this than to memorize an entire cheat sheet.
     
    Certifications: MCSE, MCDBA, CCNA, A+
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  9. ffreeloader

    ffreeloader Terabyte Poster

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    OK. I'm attaching a much more visual explanation of how I do subnetting in my head in a .doc format to this post. I knocked this out pretty quickly so if there any typo's please try to ignore them.

    HTH.
     

    Attached Files:

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  10. tripwire45
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    tripwire45 Zettabyte Poster

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    Part of me feels like a dunce for even posting the thing but I also kind of like the transaction going on here. Carry on. Don't mind me. :oops:
     
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  11. Bluerinse
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    Bluerinse Exabyte Poster

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    That's really good Freddy! But I found your following statement a little confusing...

    Does this make more sense?

    A class C address uses the 8 bits (octet) on the right for IP addresses, leaving the remaining 24 bits on the left for network ID. It is the 24 bits on the left in conjunction with the subnet mask that dictate which subnet the IP addresses belong in.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  12. ffreeloader

    ffreeloader Terabyte Poster

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    LOL. LIke I said, I knocked that out really quickly. It was posted just as I originally wrote it with no proof reading. It was "One and done." :rolleyes:

    So, good catch, Blue Rinse. That sentence should have been phrased the way you wrote it. My brain simply turns things around sometimes when I write them and I have to catch it on the proof reads.
     
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  13. ffreeloader

    ffreeloader Terabyte Poster

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    Oh, don't feel like that. I suppose cheat sheets have their uses I just don't like them personally because I'd rather be sure of completely understanding the concepts rather than relying on a piece of paper to cover my lack of knowledge.

    I know not everyone catches on to this as easily as I did, but I taught some guys I studied with to subnet in about a 1/2 hour or so after they'd been struggling with it for weeks by introducing them to this way of looking at things. Once they did it a time or two the light came on and and they couldn't believe it had taken them so long to understand the concept.

    Believe it or not the instructors were angry because I used this to teach the guys who struggled with subnetting to understand it. They thought I was doing it to show them up. It never once occured to them that I did it just to help.
     
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  14. Bluerinse
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    Bluerinse Exabyte Poster

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    That is what makes it so good, all off the top of your noggin, I like that!

    I remember the first time I sat in a class and listened to an Instructor teaching us about subnetting. It was the first time my brain totally freezed up on me. I was fine to begin with and then the fog started to rise. By the end of the class he may as well have been speaking in German. It took me a while to get it straight in my head but I got there in the end (self study) or there would have been no MCSE for me :eek:
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  15. ffreeloader

    ffreeloader Terabyte Poster

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    Well, I had to stop and think a couple of times because it's been a couple of years since I've actually done any subnetting. I'm just lucky that I can remember concepts, I guess. If I had to lean on remembering what was written on a cheat sheet for that long to do any subnetting I'd be lost, but a concept will stick with me long after memorization has faded away.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1

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