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]

Originally Posted by kevin31

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]

Subnetting Made Easy

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...