How to Subnet in your head?

kevin31

Hi Guys,

Can someone help with this the penny is just not dropping with me?

What techniques do people use?

thanks

Kev

meadIT

kevin31 wrote: »

Hi Guys,

Can someone help with this the penny is just not dropping with me?

What techniques do people use?

thanks

Kev

Here is my plan for taking the exams. Before I start the test, I'm going to scribble down the following table to help me do the subnetting in my head.

2^1  2
2^2  4
2^3  8
2^4  16
2^5  32
2^6  64
2^7  128
2^8  256
2^9  512
2^10 1024
2^11 2048
2^12 4096

128  /9   /17 /25
192  /10 /18 /26
224  /11 /19 /27
240  /12 /20 /28
248  /13 /21 /29
252  /14 /22 /30
254  /15 /23
255  /16 /24

The first table will allow me to do powers of 2 quickly. The second table will allow me to convert from CIDR to decimal notation quickly. Using these two tables should allow me to do everything else in my head.

Examples:
Which subnet does the host 192.158.4.75/22 belong to?

A: Convert the /22 notation to decimal subnet mask using the table: 255.255.252.0. The network blocks are going to be multiples of 4 (256 - 252 = 4), so the networks will be 192.168.0.0, 192.168.4.0, 192.168.8.0. Answer is 192.168.4.0

What are the available hosts in the above subnet example?

A: We know that the network address is 192.168.4.0, so the first host would be 192.168.4.1. We also know that the next network will be 192.168.8.0, so 192.168.7.255 will be our subnet's broadcast address, leaving 192.168.7.254 as the last available host. Answer is 192.168.4.1 - 192.168.7.254.

Neeko

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

Learn that simple method and with an hour of practice you'll soon be doing them in your head.

mattrgee

Practice

laidbackfreak

practice..... practice...... practice..... I did around an hour each day on various sites till I cracked it (or found a method that worked for me)

seriously tho do you understand the concepts ? where are you struggling with ??

If you want some facetime gimme a shout Im happy to hook up sometime and help out if you want ?

Kaminsky

Start by working in binary until you get used to what is going on and then go to decimal and powers of 2. Everything else will follow. It may seem long and loborious way of doing it but, it will click much faster. It will also stay in your head longer this way.

Search back over many months of CCNA/CCENT forum for many examples and clues.

Fine tune your skills at *******************.com over and over and over and then that's when you start getting to be able to do it in your head at very fast speeds. Standard question shouldn't take longer than 10 seconds leaving you 80 seconds per question to spend elsewhere in the exam.

Edit: didn't know subnetting questions was a bad site ? Is this correct ?

aordal

Nah the website is fine, dunno why the filter is blocking it.

dynamik

I believe they've spammed here.

tiersten

Unfortunately there is no magic trick behind it. You just have to practise lots. Either that or get the various tables tattooed onto the insides of your eyelids. I guess it'd have to be some sort of glow in the dark ink as well...

pleasehhelp

i have a few subnetting questions for you. please reply back at me.

thanks,

Monkerz

meadIT wrote: »

128  /9   /17 /25
192  /10 /18 /26
224  /11 /19 /27
240  /12 /20 /28
248  /13 /21 /29
252  /14 /22 /30
254  /15 /23
255  /16 /24

A: Convert the /22 notation to decimal subnet mask using the table: 255.255.252.0. The network blocks are going to be multiples of 4 (256 - 252 = 4), so the networks will be 192.168.0.0, 192.168.4.0, 192.168.8.0. Answer is 192.168.4.0

What are the available hosts in the above subnet example?

A: We know that the network address is 192.168.4.0, so the first host would be 192.168.4.1. We also know that the next network will be 192.168.8.0, so 192.168.7.255 will be our subnet's broadcast address, leaving 192.168.7.254 as the last available host. Answer is 192.168.4.1 - 192.168.7.254.

When I took ICND1, my CIDR table was a bit different. It took a few seconds less time, because I didn't have to subtract to find the increment.

/17--/18---/19--/20--/21--/22--/23--/24
/25--/26---/27--/28--/29--/30--/31--/32

128--192--224--240--248--252--254--255

128---64---32---16----8----4----2----1

I am finally to the point where using a chart is actually slower than doing it in my head. Practice does make a difference.

chmorin

Practice and memorization for me.

gosh1976

I use the second table that this guy uses and a variation on his method. Creating Your Own Mental Subnet Calculator

I don't write down the 1st table but I do write down the powers of two in a single column.

The key is to practice, practice, practice.

mikej412

pleasehhelp wrote: »

i have a few subnetting questions for you. please reply back at me.

This is an Internet Forum -- if you have a question that's relevant to this thread POST IT.

Then the next available member who has the time and an answer will get back to you.

And while we may help you with your homework -- we're not going to do it for you.

chmorin

mikej412 wrote: »

This is an Internet Forum -- if you have a question that's relevant to this thread POST IT.

Then the next available member who has the time and an answer will get back to you.

And while we may help you with your homework -- we're not going to do it for you.

Plus that way, everyone can benefit from the experience. That is why forums rock.

headshot

Look at the numbers and determine the relationships behind the numbers. If you don't understand the relationship (the why) then you wont know why you are performing whatever calculation the question asks. Don't rely on any other magical formulas and stick to the basics. That's my tip.

sthompson86

Hello I did not want to make a new thread on this, and if the OP does not mind this thread is a great segue for my question.

So I am now onward with learning how to subnet, and obviously every book/video has its own way of teaching sub-netting.

I personally found Jeramey in CBT Nuggets way of explaining sub-netting to be the most beneficial to me. I also have Wendell Odom's ICND1 book, and I understand his methods, but his way seems a bit laborious.

My question is should I stick with what works for me? or try and view as many versions of sub-netting I can? Personally, I am for just focusing on Jeremeys CBT sub-netting videos and skipping the entire Subbing chapter in my Cisco ICND1 book, for the book just confuses me.

- Thanks in adv

hermeszdata

sthompson86 wrote: »

Personally, I am for just focusing on Jeremeys CBT sub-netting videos and skipping the entire Subbing chapter in my Cisco ICND1 book, for the book just confuses me.

Personally, I would not recommend this. What I would recommend is using additional resources in addition to what you already have. Learn the process each author "suggests" and then figure which works best for you. The process I use is actually a melding of 3 different processes.

There are times where one process is more suitable than another. This is why a complete understanding of subnetting is so important.

John

sthompson86

hermeszdata wrote: »

Personally, I would not recommend this. What I would recommend is using additional resources in addition to what you already have. Learn the process each author "suggests" and then figure which works best for you. The process I use is actually a melding of 3 different processes.

There are times where one process is more suitable than another. This is why a complete understanding of subnetting is so important.

John

Ok that makes sense. That is how I generally prep for an exam. I read multiple books etc. Thank you.