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Subnetting class A

Everest63

Hi all,
This one has me stumped but I have a feeling that once I get the answer/ reason I'll be hitting myself on the side of my head with a "duh!"

On the tutorial called "Subnetting Made Easy" there is this question.

What is the valid host range of the 7th subnet of address 10.0.0.0 /14 ?
The block size is 4, from 16-14 = 2 then 2 to the power of 2 = 4. We are borrowing in the second octet so count in the
block size from 0 seven times to get the seventh subnet.
The seventh subnet is 10.24.0.0. Our valid host range is is 10.24.0.1 to 10.27.255.254 again remembering not to include our subnet 10.24.0.0 and the broadcast address 10.27.255.255

10.0.0.0
10.4.0.0
10.8.0.0
10.12.0.0
10.16.0.0
10.20.0.0
10.24.0.0 7th subnet!
10.28.0.0
10.32.0.0

My question: How do you get the 27? Wouldn't 27 be the broadcast number?
To get the broadcast number for 10.24.0.0 you would go up to the next subnet (10.28.0.0) and subtract 1 to get (10.27.0.0) and this would be the broadcast. To get the host then subtract 1 again to get 10.26.0.0, or .....10.26.255.254 ?

Or, is my reasoning above incorrect? Maybe this?
Since the 4th octet is a 254 to avoid the broadcast of 255, then the broadcast of .27 and .255 in 2nd and 3rd octets can stay put as the 4th octet is avoiding the broadcast?

Andrew

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Comments

carterw65

This helped me in a big way:

The Art of the Subnet **** Sheet And Other Subnetting Tips

carterw65

Breakdown:

10.24.0.0/14 is

10.24.0.0 255.252.0.0 (Network)
10.24.0.1 255.252.0.0 (First Host)
10.27.255.254 (Last Host)
10.27.255.255 (Broadcast)

clarson

your right that the next subnet is 10.28.0.0. But, you are not subtracting the 1 correctly. 
when you subtract the one, you do it from the whole address. in binary 
0000 1010 0001 1100 0000 0000 0000 0000 10.28.0.0 
-1 
0000 1010 0001 1011 1111 1111 1111 1111 this is the broadcast address 10.27.255.255 
-1 
0000 1010 0001 1011 1111 1111 1111 1110 this is the last host address 10.27.255.254 
 
the broadcast has all the host bits set to one. so 32-14=18 so the last 18 bits are set to 1 
the 27 comes from the 7th subnet. 7-1=6 (-1 for the zero subnet) so the subnet bits are 000110 and the last two bit are 1 because the for the broadcast address all the host bits are set to 1. that makes the 2nd octet 00011011 = 27 
 
what your doing is similar to in decimal 
101 000 
-1 
100 900

james43026

If you truly want to understand how subnetting works. I recommend looking into the binary side of subnetting, it is truly the only way you are going to have a real understanding of what is happening. Looking at if from the perspective of subtracting numbers for the dotted decimal values just lands you into more confusion and trouble later on down the line.

cmztech

My question: How do you get the 27? Wouldn't 27 be the broadcast number?
To get the broadcast number for 10.24.0.0 you would go up to the next subnet (10.28.0.0) and subtract 1 to get (10.27.0.0) and this would be the broadcast. To get the host then subtract 1 again to get 10.26.0.0, or .....10.26.255.254 ?

Or, is my reasoning above incorrect? Maybe this?
Since the 4th octet is a 254 to avoid the broadcast of 255, then the broadcast of .27 and .255 in 2nd and 3rd octets can stay put as the 4th octet is avoiding the broadcast?

Your second reasoning is closer, but I think I see where your logic is tripping you up, you have to remember your valid host include 10.24.0.1 - 10.27.255.254. So your broadcast number is ultimately the number after the last usable host in your subnet, i.e. 10.27.255.255 is the broadcast for that subnet.

Remember don't miss out on all the host available in the 10.27.0.0 - 10.27.255.254 range. They want some love, too!

cmztech

Allow me to slightly correct my words, I just said your second reasoning is closer in the Quote there, but what I really meant is that overall you were close. Your first reasoning has some accuracy to it, also.

Yes, you do go to the next subnet 10.28.0.0 and subtract 1 to get to 10.27.0.0. However, 10.27.0.0 is not your broadcast, that is simply the last number that is allowed in the "interesting octet" (second octet in this scenario thanks to it being a Class A with CIDR /14), before it enters the new subnet of 10.28.0.0. So, after you did you subtraction, the next thought is "ok, what are my usable addresses?" The answer to that includes the host bits in the 10.27.0.0 -10.27.255.254 range.

-different people may think of it in different ways
-ultimately like James has mentioned, make sure to go back to the binary view of it so you can see how the bits are really affecting the numbers
-I hope I did not add to the confusion. Subnetting is fun, so I tend to carry on about it.

Everest63

Thanks all for your replies! I am studying for the CCENT using Todd Lammle's CCENT 100-105 book. I also bought Odom's book. It's kind of dry but I like the detail it goes into. I am fine with class C subnetting, but when I try subnetting questions that are class A & B I can at times get tripped up. Jeremy over at CBT Nuggets is heavy into binary and I like that. He says binary takes longer but it's invaluable for subnetting A & B addresses so you know exactly where you are and what is happening. So I will use binary in my subnetting and hopefully by exam time I will be able to do the less complex subnet questions in my head.

8thdegreepwnologist

A broadcast address is the last available IP address within a subnet

10.27.255.255 = is the broadcast address for all hosts on subnet 10.24.0.0
255.255.255.255 = is the broadcast address for any host on any reachable subnet

rob42

Hi Andrew,

I agree with the comment here that understanding the binary bits will give you a very good understanding about the 'nuts and bolts' of what's going on. I'm not sure if I'm permitted to post a link to an app that I use (it's a Android OS app), but it's called Subnet Quick. It's VERY good and it's given me a very clear understanding as it displays a lot of information in a very clear way.

I like the the Subnet Calculator app that I got from this site, but (IMHO) it would be better if it also displayed the binary notation.

All the best with your studies.

Rob.

Edited to add…

I’ve just discovered that the Subnet Calculator app that I got from this site does indeed show the binary bits by clicking on the ‘Network address:’, ‘Broadcast address:’ or the ‘Valid host range:’

My thanks to TechExams.net for this fabulous app!

nastynate604

Hi Andrew,

Just thought I would throw my two cents in here. I originally obtained CCENT 5 years ago. Subnetting was brutal for me because I am not the greatest at math. All of the methods I learned to subnet seemed overly complicated. I like things to be pretty simple. In short it probably took me 3 weeks to fully grasp subnetting the first time around. This time around again I was missing certain things. I found this video on Youtube by this guy named Laz. His method made something in my head click and after that I was subnetting in my head after only a couple nights of practicing. It's funny but after he explained it and I used his method for a few nights, I went back to reading Lammle and I found I liked his method even better. Anyways, check this out.

https://www.youtube.com/watch?v=rs39FWDhzDs

Cheers,

Nate