Subnetting - Chapter 4, Page 161

[army]

Hi all



I'm up to:
- Chapter 4
- Page 161


And here is a question:
- "If your host ID is 10.20.80.30, what is your subnet, broadcast address and valid host range"


- The next paragraph then goes onto say:
"The subnet in the second octet is 20 with a block size of 1, but a third octet is in block sizes of 16, so we'll just count them out: 0, 16 etc .. .. "



So my questions are:
- How is the second octet a block size of 1? I thought this was only a Block size of one if the octet was all masked/all 1's?
- How did you find out the third octet (.80), is a block size of 16?


Greatly appreciate any answers or help (i'm very very new at this, the more lengthy posts, the better! :D)


Thank you all

[army]

not sure where your reply went, ciscodaze..

Anyway, the chapter doesn't really say which Subnet Mask it is using.


Also not sure if I can quote more of the text book,


But here it goes, this is all from Page 161, Chapter 4 in CCNA Exam 640-802, Seventh Edition
You only need to worry about the octet that has the largest block size (typically called the interesting octet; one that is something other than
0 or 255)—for example, 255.255.240.0 (/20) with a Class A network.

The second octet has a block size of 1, so any number listed in that octet is a subnet. The third octet is a 240 mask, which means we have a block size of 16 in the third octet. If your host ID is 10.20.80.30, what is your subnet, broadcast address, and valid host range?

The subnet in the second octet is 20 with a block size of 1, but the third octet is in block sizes of 16, so we’ll just count them out: 0, 16, 32, 48, 64, 80, 96…voila! (By the way, you can count by 16s by now, right?) This makes our subnet 10.20.80.0, with a broadcast of 10.20.95.255 because the next subnet is 10.20.96.0. The valid host range is 10.20.80.1 through 10.20.95.254. And yes, no lie! You really can do this in your head if you just get your block sizes nailed!

[ciscodaze]

Deleted it, sorry.

Ok, as I thought, a mask was provided. 255.255.240.0 (/20).

All he means by saying the second octet has a block the size of 1, is that we are not dealing with that octet all all ( but an awkward way to explain it).

He is just trying to clarify that since we are subnetting the classful 10.20.80.30 network (meaing we started out with the classful mask of 255.0.0.0) and are now applying a mask that changes the third octet, that that is where the action is...the third octet.

As he said, just focus on the octet that is neither a 0 or a 255 (often called the "interesting octet"). Thats .240 in this case.

To get the block sizes to figure out the subnets, just do 256-240=16.
Your block size is 16, and just remember, you always take the number in the "interesting octet" and subtract it from 256 to get your block size. Always. That way, you can do this in your head quite easily.

Then once you have your block size for that octet, start counting by it
(16 in this case). Stop at 80 (5x16) because 80 is part of the "80-96" block.

IF the number supplied in the third octet had been 79, (just as another example) you would have stopped at 64, (because 79 is between 64 and 80) leaving you with subnet 10.20.64.0, and a valid host range of 10.20.64.1 - 10.20.79.254, with a broadcast of 10.20.79.255

hope that helps.

you need an online calculator to clarify things whe you get confused.
This is one of many http://www.subnet-calculator.com

[army]

Deleted it, sorry.

Ok, as I thought, a mask was provided. 255.255.240.0 (/20).

All he means by saying the second octet has a block the size of 1, is that we are not dealing with that octet all all ( but an awkward way to explain it).

He is just trying to clarify that since we are subnetting the classful 10.20.80.30 network (meaing we started out with the classful mask of 255.0.0.0) and are now applying a mask that changes the third octet, that that is where the action is...the third octet.

As he said, just focus on the octet that is neither a 0 or a 255. Thats .240 in this case.

To get the block sizes to figure out the subnets, just do 256-240=16.
Your block size is 16, and just remember, you always take the number in the "interesting octet" and subtract it from 256 to get your block size. Always. That way, you can do this in your head quite easily.

Then once you have your block size for that octet, start counting by it
(16 in this case). Stop at 80 (5x16) because 80 is part of the "80-96" block.

IF the number supplied in the third octet had been 79, (just as another example) you would have stopped at 64, (because 79 is between 64 and 80) leaving you with subnet 10.20.64.0, and a valid host range of 10.20.64.1 - 10.20.79.254, with a broadcast of 10.20.79.255

hope that helps.

you need an online calculator to clarify things whe you get confused.
This is one of many http://www.subnet-calculator.com
Thank you very, very much for taking the time to post the above.

Makes much more sense now, greatly appreciate that Daze

[lammle]

I am confused on the confusion myself.
It says "255.255.240.0 (/20)" on the first line, which is the mask.
Todd Lammle