Everything you need to know about IP Subnets.

I’ve been doing support for a long time and I’ve configured network settings repeatedly over the course of that career, but if I were to be completely honest I’d have to admit that I haven’t a full understanding of how the TCP/IP protocol works in general or subnets in particular. I just know that some things need to be set up a certain way and everything works. It’s one of those things that I’ve been meaning to get around to teaching myself someday, but never have.

Then I stumbled across IP subnetting made easy by George Ou and I can now die a happy man:

IP subnetting is a fundamental subject that’s critical for any IP network engineer to understand, yet students have traditionally had a difficult time grasping it. Over the years, I’ve watched students needlessly struggle through school and in practice when dealing with subnetting because it was never explained to them in an easy-to-understand way. I’ve helped countless individuals learn what subnetting is all about using my own graphical approach and calculator shortcuts, and I’ve put all that experience into this article.

Granted, this is probably way more technical than a lot of you care to get, but I know I have a few fellow support people who read SEB and they may be in a similar boat of not really ever having bothered to learn subnetting so I thought this might prove useful to them. Plus it acts like a bookmark for my future reference.

Hat tip to SunbeltBLOG.

19 thoughts on “Everything you need to know about IP Subnets.

  1. Cool!  I re-wrestle with subnetting every six months or so for one reason or another.  The article lets you save a .pdf, making it my lunchtime read tomorrow.

  2. Both sites aren’t bad, but the best way to learn subnetting is to spend a half hour to an hour with someone who knows it.  I have read books and been to websites, and all I needed was one class period with a teacher I had a year ago.  And I still know it today.

    Understanding subnetting helped me on about every certification exam I have sat for.  I haven’t passed that many certification exams, but I know for certain it wasn’t because of subnetting.

    Sepharo, if you need any help let me know, either comment or email me.  I have helped dozens of students using very simple but effect techniques that make understanding this subnetting thing a breeze.

  3. i created my own complete subnetting spreadsheet about 8 years ago.  i only have a hard copy i’ve been dragging around with me since then.  i need to recreate it back in soft copy so i can pass it around.  i’ll post it back here if anyone would like it.  it’s a great cheat sheet to have.

  4. When I was working on my CCNA I had to of course learn about subnets etc. It was confusing at first, but like a lot of things involving numbers once you get it, it sticks.

    The worst part for me was converting decimal IP addresses to binary, then to hex, then back to decimal. That came up on the CCNA test and about drove me bananas.

  5. Man I wish I had got that question on the CCNA.  The simulators tore me apart.  I only had two but by the time I finished the second one I had only 22 minutes left and had about 13 or 15 questions to go.  This worried me cause I wasn’t sure if I would get another simulator or not.  And I was doing so damn goo up till that point.

  6. “The worst part for me was converting decimal IP addresses to binary, then to hex, then back to decimal. That came up on the CCNA test and about drove me bananas.”

    Jeff, this is exactly why I teach you NOT to do the DEC to BIN so you can run AND and then convert BIN back to DEC.  When I tell people they can go ahead and run the AND operator on decimal numbers directly using the Windows Calculator, they almost feel like they’ve been the victim of a sick joke by their networking instructor/book.


    George Ou
    Author of “IP Subnetting made easy”.

  7. Yo Webs,

    I explained in my article that you don’t need to do binary conversions smile.  I’ve never understood why teachers torture their students with the binary conversions since you don’t need to convert to binary to use the AND operator in Windows Calculator.  Windows Calculator lets you run AND directly on decimal numbers so why convert to decimal before you do the AND and then convert the result back to decimal?

  8. The reason why most students are taught to convert Binary <-> Decimal, Decimal <-> Hexadecimal, Hexadecimal <->

    Binary is because a lot of different certifications require that knowledge.  Some examples, CCNA, A+, N+, MCSE, and so on.

    At least that is why we were taught how to do it.  But really, once you learn it is all incredibly easy.  I could probably teach anyone how to do it in a matter of minutes.  At least anyone with some computer knowledge. wink

  9. Back in the olden days I used to have to number base conversions all the time.  One example was that a piece of mid-‘70’s vintage electronic equipment that I worked on was programed using DIP (binary) switches, but the codes that I was given were in HEX. 

    I used to have to boot computers using binary front panel switches, and the op codes that I needed to enter were either in Octal.

    There are many other examples of where I had to do base conversions.  Kids now-a-days don’t know how easy they have it.

  10. I’ve never understood why teachers torture their students with the binary conversions

    There’s learning by rote (as in plugging numbers into a calculator or looking up a spreadsheet), as opposed to understanding the theory and being able to do it “by hand”. If you can do the latter, take all the shortcuts you want; until then, doing it the “hard” way builds character.

    IP subnetting is easy enough anyway—unless you use a non-contiguous network mask for studly sport.

  11. elwedriddsche,

    I’m well aware of the need for theory and I do try to fully explain the theory along with graphical representations.  What I do not understand is that once you have the students learn the concept of binary conversions and masking, you need to move on and teach them the easy way of using the AND operator on decimal numbers.

  12. I have heard of using AND on decimal numbers – I have NEVER seen a use for that, I would convert the numbers to binary, AND, convert back to decimal—and wonder: Why in the hell?

  13. Itdontmatter,

    Why convert to binary, use the AND operator, convert back to decimal, then find out what the Network ID is.  Do the AND on the decimal directly.

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