Question

# I guess I actually don’t understand subnetting...

I’m studying for my Net+ exam and I know for a fact that I’ll need to understand subnetting enough to get the following from an IP address with CIDR notation:

I thought that I understood this enough to find these (and I’m typically successful. My main concern is that about a quarter of the random problems I practice with give the most absurd answers for the IP address range while the other three quarters work out perfectly. This means (as far as I understand) that getting the network, broadcast, and number of addresses correct is a real problem... I’m really having difficulty finding help myself because everyone seems to have a different method and/or leaves out information that they assume the reader knows/is aware of.

Here are two examples to illustrate my point. The first comes out as it should while the second is a mystery. Hopefully a network wizard out there can show me the error of my ways. Please explain it like I’m a simpleton and the less binary-decimal conversions, the better.

IP: 192.168.1.1/26

11111111.11111111.11111111.11000000
Meaning...
255.255.255.192

We get ‘192’ by adding the two 1s (128+64=92). Ok, then with the subnet mask in binary form, we can get the broadcast address by adding the 0s in the last octet (32+16+8+4+2+1=63)

So, the broadcast address is 192.168.1.63 making the upper limit of the IP address range 192.1681.62, correct?

Here’s where my understanding gets a little murky but still works... sorta...

The initial IP address is the lower limit of the IP address range, meaning that the network address is one below this at 192.168.1.0. I don’t know where I got this idea (about how to get the lower limit) but it is correct for nearly all problems that I’ve tried. The fact that it’s not always right has me worried...

Anyway, knowing the network and broadcast address gives us the number of hosts at 62 (63-1=62). I’ve checked with calculators that agree with all of these answers.

Now here’s one that doesn’t follow these rules for reasons that I cannot understand.

IP: 192.168.10.44/28

Subnet mask: 11111111.11111111.11111111.11110000 = 255.255.255.240 (128+64+32+16=240)

Like before, I add up the remaining ‘0’s from the last octet to get 15 (8+4+2+1=15) which gives a broadcast IP address of 192.168.10.15. This is apparently not correct as the real answer is 192.168.10.47.

Obviously there’s something I’m not doing correctly as these rules don’t always apply. Maybe it’s just coincidence that has my first method giving correct numbers, but I can’t be sure. All help is appreciated but might I humbly request that we try to correct where I’ve gone wrong in the system used above vs reinventing the wheel? I very much dislike having to convert between decimal-binary more than I have already and, while I understand that most methods require it, I would like to stick to simple rules for the test. This is, of course, unless I was lied to and it is absolutely unavoidable.

Help plox...?

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Clarification Request

If you're unable to answer these questions, there is no sense in pointing out the multitude of tutorials out there. It's a minor miracle that I understand just how to get the subnet mask, so there's no sense in being lazy and pasting links to more tutorials that have already not helped me.

Does anyone actually have a firm enough grasp of this to tell me where my methodology goes wrong and how to fix it?

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Clarification Request

If you're unable to answer these questions, there is no sense in pointing out the multitude of tutorials out there. It's a minor miracle that I understand just how to get the subnet mask, so there's no sense in being lazy and pasting links to more tutorials that have already not helped me.

Does anyone actually have a firm enough grasp of this to tell me where my methodology goes wrong and how to fix it?

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There are far too many tutorials on this.
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Thank you, but this certainly do not answer my questions.

In reply to: There are far too many tutorials on this.

Like I mentioned, I've been unable to find an explanation that is anything close to what I've been able to understand so far. This is because every explanation seems to be a different approach and it is always assumed that the reader knows things that are critical to that specific method. I don't need incomplete explanations, but I do need someone to say where I've gone wrong with this one.

I really do appreciate the effort you've made to type a few lines of not help and for providing a link to a website that does all of this for me. Unfortunately, I will be required to do this by myself and this means not using another random subnet calculator.

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Once in a while a member wants to hold classes where they ask what is on the web to be copied and repeated here. That will take someone willing to do this. This text box is only so big so to go over the book your class is using sentence by sentence and the back and forth that you are getting it at each sentence means I am now a tutor.

So this is my answer: Either read and watch more tutorials or find a tutor in your area for this. Forums are a poor way to do tutoring work.

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