Question regarding subnets (Studying for CCNA)

Pheonyx

Hi everyone,

I've ran into a problem while studying for my CCNA exam, and hoping someone kind enough to shed some light on it.

The problem asks to find the subnet ID with only the IP address residing in it and its associated mask.

IP 10.220.100.18

Mask: 255.255.128.0

The book (Official Cert Guide Cisco CCENT/CCNA ICND1 100-101 Wendell Odom) teaches to use the magic number method, which states, "To find the subnet ID, simply find the magic number (256 - Mask), then find the multiple of the magic number that is closest to the IP's octet without going over it and place that in for the subnet ID.

So in this case, magic number = 256 - 128 = 128. The problem is that 128, is already higher than 100, so I'm left pulling hairs out of my skull trying to figure out why this is happening.

I'd appreciate any and all help, I'm at my wits end here and very frustrated....thank you!

-Pheonyx

Pheonyx

I just found out why and to anyone reading this that ran into the same problem, its simply lack of attention to detail.

We have to start at 0 when calculating the multiples of the magic number, stupid mistake.....I think I need a break....my brain feels like its going to implode lol

thomas_

With 255.255.128 subnet mask you only have two subnets: 10.220.0.0 and 10.220.128.0

The ip address 10.220.100.18 falls within the range of the 10.220.0.0 subnet. If that is exactly how that rule is listed then it's not completely accurate as it doesn't mention the first subnet.

Pheonyx

yeah it does mention that, i just overlooked it. thank you thomas!

HAMP

Well!!!
Whats the answer to the question, which subnet does it belong?

Pheonyx

Heres the answer I got, and I checked it on a subnet calculator so hopefully its right:

IP: 10.220.100.18
Mask: 255.255.128.0 /17
Sub ID: 10.220.0.0
Broadcast: 10.220.127.255
Available Host Range: 10.220.0.1 to 10.220.127.254

I find that the harder part now is memorizing all the power's of 2 when you are calculating the number of subnets and host addresses. like 2 to the power of 23 or something like that is pretty hard at first.

HAMP

Looks good to me....

After awhile it will become easier then knowing your timestable.

Btw,

If you gotten your answer without the subnet calculator, then I have to say, good job.

Pheonyx

Scout's honor haha I definitely did it on my own, or else I would be shooting myself in the foot come exam time.

and that's good to know that it will get easier, because at the moment, it kind of looks like rocket science to me, although much more interesting than rocket science (to me anyways).

alias454

I would highly recommend watching this free video - it's worth your time. https://www.udemy.com/ip-adressing-and-subnetting/?dtcode=5arWhaz37C7c
You can probably skip the first section and go right into section 2 and then watch the rest of it.

Pheonyx

Thank you Alias454! I will definitely watch it.

I have another question for you guys since I have your attention:

I was talking to a friend of mine that is working as a network engineer now, and he says taking the icdn1 and 2 separately makes you look bad when an employer see's that you separted the exams into two parts rather than taking them as one composite exam. is this true?

alias454

I cannot speak from an employers perspective but I can give you an answer if one ever asks you about it. In my opinion, the two test route forces you to answer more questions because you are sitting for two tests. 50-60 questions x2 vs. the one test route, which is 50-60 questions x1. Granted, taking one test at a time, allows you to focus on a smaller subset of topics but shouldn't be looked down upon IMHO. I am sitting for the 200-120 test this coming Thursday. I decided on the single test, while I have to be able to answer from a broader pool of questions, statistics are on my side (I don't plan on leaving it up to luck of the draw but I think you get where I am going). In either case, you have to know the material or else you don't pass. Just my .02.

Regards,

Pheonyx

Much appreciated sir goodluck on your exam!

Jon_Cisco

subnetting.net - Subnet Questions and Answers

I always recommend this site for subnetting practice. After about an hour of practice it will all start to make sense and become much faster.

Good Luck!

happyend98

Check out the Networking Doctors site( The Networking Doctors ) and watch some of the Lazaro Diaz stuff. He's a CCNA mastermind and fun guy to learn from. You will appreciate his humor.