Odom's 100-101 subnetting methoderrors?

bbarrick

I just started subneting, I think I'm pretty good at it for the most part, using binary math I can figure it out rather quickly. But I just started reading Odom's method with the interesting octet/magic number. If anyone has his new book on 100-101, please tell me pg. 384/385 with fig. 14-8 is just fraught with errors on the subnet masks. It makes the entire explanation quite confusing until you actually see it done on his video.

I'm not sure if it would violate forum rules to upload a screenshot or quote verbatim from the book, but hopefully someone can confirm this.

Ismaeljrp

I love subnetting.

I dont have the book...but i understand the method he uses.

The magic number is only meant to find the subnet increment.

If you are given a mask of 225.225.192.0

The third octet is were the reverse engineering starts ( significant octet ).

Now how do we know the hop ? Subtract that octet from 256 . 256-192=64 , your subnet range is 64 addresses. Subnet range = increment.

Ismaeljrp

I dont like the method as it is slow.

Just think, the increment = lowest network bit (in its decimal form)....

Since u are good with binary this should be easier.

In this case you still only work with the significant octet which is .192

Where is the lowest bit for 192 in its binary form ?

128, 64, 32, 16, 8, 4, 2, 1
1 1 0 0 0 0 0 0
^
^ this is your lowest network bit, and as such it is also you increment ( subnet range )

Ismaeljrp

Sorry im on my phone..
64 is the lowest bit

bbarrick

Thank you, I figured out what his method by watching his videos. I'll try to paraphrase his explanation in this book and see what you think.

The following figure will show four different masks being considered using in a Class B network 172.16.0.0. The figure will show the third octet value for the subnet IDs that would be created when using mask 255.255.255.128, 255.255.255.192, 255.255.255.224, and 255.255.255.240, from top to bottom.

Figure:
Subnets of 172.16.0.0: 172.16. ____ . 0

255.255.128.0 0 128
2 subnets |

---

|

---

|

Then it goes on to show three more based on:

255.255.192.0
4 subnets

255.255.224.0
8 subnets

255.255.240.0
16 subnets

Then in the paragraph following the figure it talks about if you use 255.255.255.128 as the mask, the mask creates two subnets with ID's 172.16.0.0 & 172.16.128.0. and so on with the other three but he lists in this paragraph the masks he's not using in the figure but the masks he used in the previous paragraph.

The way I'm looking at it, between what he's writing in the paragraphs and what he's got in the diagram I see 8 different subnet masks being used.The four he's talking about and the four he's using in the figure.

I've caught a couple of really simple things in this book so far, and reported one of them that didn't show in the errata list, so I was trying to figure out if this was in error as well or I'm just not understanding the books explanation of it.

Ismaeljrp

Having a glance ( my phone is dying ). I say those are errors.

Not possible , the subnet IDs that /25 creates cant change the third octet in the IPs. That mask gives 172.16.0.0 and 172.168.0.128 subnets....and also he used the Network ID itself as a subnet if that were the case.

flattley

I can see how you find it confusing if he is using material from previous paragraphs.

A /25 is a mask of 255.255.255.128 and would indeed only give you to subnets. The 172.168.0.0 and 172.168.0.128 would be the correct subnets - as stated by Ismaeljrp. I have found that masks and subnets are used interchangeably - perhaps causing confusion.

I think if you do a handwritten subnet calculator you won't go wrong. I always work with the interesting octet to find the range. I find it better for me that way.

All you need to do is subnet the interesting octet from 256 to get your block size

Souljacker

He might even be mixing material from his older version of the book. The subnetting is something that I expect he wouldn't have had to change from one book to the next, but going back over my copy of his ICND1 book for the current (being retired) test, I see no such ambiguity in the explanation.

Have you tried asking on his Facebook? He's actually pretty good about getting back to his readers.

bbarrick

Souljacker wrote: »

He might even be mixing material from his older version of the book. The subnetting is something that I expect he wouldn't have had to change from one book to the next, but going back over my copy of his ICND1 book for the current (being retired) test, I see no such ambiguity in the explanation.

Have you tried asking on his Facebook? He's actually pretty good about getting back to his readers.

No, I haven't. Thanks for that suggestion though, I'd forgotten that I had liked his page there. I will do that.

bbarrick

How do you guys handle the higher end of the powers of 2? I was looking at a question in the book and the number of subs was 2^22 power. Did you memorize them, because I wouldn't want to waste the time trying to calculate that during the test.