(Practicality)How I plan to subnet during the test...

Michael.J.Palmer

All right, this is what I've been doing to properly subnet in labs and what not within a fairly quick amount of time (really quick actually). I just wanted to share it with everyone and find out if you have any additional suggestions.

Something to keep in mind:
The testing site I'm testing at gives us a dry erase sheet to use during the exam to keep track of notes and what not.

Basically I'll go in, sign in for the exam, they hand me the dry erase board. I'm escorted to my seat in the testing room, sit down to take pre-test assessment, whatever might be there. During this time I take the time to fill out information off the top of my head onto the dry erase sheet to help with subnetting. Here's what I have memorized and how I have it memorized.

Slash Notations:

/32 = 1 host
/31 = 2 hosts
/30 = 4 hosts
/29 = 8 hosts
/28 = 16 hosts
/27 = 32 hosts
/26 = 64 hosts
/25 = 128 hosts
/24 = 256 hosts etc.

Basically I just start with /32 and 1 and as I work down I double the number, sounds easy enough right? This is how I remember how many host machines I can have in a given slash notation, keep in mind I know to subtract 2 to get the exact number, but I don't do that in this case for another reason. I'll get into that in a minute, first the binary configuration/figuring out the long 255.255.255.0 mask of my subnet that I'm sure we all know.

I plan to write out the following:

128 64 32 16 8 4 2 1

At that point in a case where let's say the question has me subnetting a 192.168.1.0 into a /27 and figuring out the network addresses and subnet mask of them. To figure out the IP subnet version I'll just remember that /24 is my base of 255.255.255.0 so I'm borrowing into the last octet, then I'll do a quick addition of 128 + 64 + 32 (which I already know off the top of my head as 224 tbh with you) so my IP mask is 255.255.255.224.

Ok, so onto the reason why I keep the hosts numbers complete, this is how I figure out my network addresses easily.

I'm keeping in mind the number of total hosts I plan to utilize to figure out the number of networks I can have/the exact address of each network.

Let's take for example /27 again, it's 32 hosts. Well in this case my networks are simple. I start with:

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

I added 32 to end of every network ID to get my next network ID.

I stop at 224 because my mask ends with 224.

That's my answer for the network IDs for each network, the total number of networks I have is just the total count that I have so I have 8 networks total.

If I wanted to check this number I could multiply 8 and 32 together, since /24 base gives a total of 256 hosts then 8 * 32 should equal 256 and does.

I'm very good at these math calculations and can do them in my head fairly quickly. Does this make sense to anyone else but me? Do you know of an even shorter hand version to do this? I'm sure there are several but the idea here is to try and find a way to get multiple aspects of the same questions within a minute or less.

Suggestions are appreciated, thanks.

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Comments

earweed

Sounds like you've got a good system that works well for you and that's what counts.
Good luck on your test!

Eildor

That's pretty much the same way I do it

Michael.J.Palmer

Thanks for the encouragement. I've just realized that there are half a million ways for someone to do subnetting and just wanted to see if anyone else ever did this and what not. Guess I'm just looking for confidence boosters before I test this evening (hopefully, still haven't heard from the center but I'll find out around 6:00 PM tonight).

fslima0

Michael.J.Palmer wrote: »

Thanks for the encouragement. I've just realized that there are half a million ways for someone to do subnetting and just wanted to see if anyone else ever did this and what not. Guess I'm just looking for confidence boosters before I test this evening (hopefully, still haven't heard from the center but I'll find out around 6:00 PM tonight).

That's how I do it too. I just don't write it out on a piece of paper. I do it on top of my head. Good luck.

earweed

I just looked back at my subnetting notes from when I studied subnetting for the 70-642 and that's how I did it then. I'm doing most of it in my head right now though. I'm getting ready for the CCENT and plan to take it as soon as I have the $$ to spare for the test.

fslima0

earweed wrote: »

I just looked back at my subnetting notes from when I studied subnetting for the 70-642 and that's how I did it then. I'm doing most of it in my head right now though. I'm getting ready for the CCENT and plan to take it as soon as I have the $$ to spare for the test.

It's better to learn the binary way first, and then after that, you can create shortcuts. It worked for me that way because it makes more sense.

-Foxer-

I do basically the same thing as well.

Futura

Everytime I shower I do my 8x 16x 32x and 64x tables and which cidr notation they relate to.

Just thought i'd share that info with you...it's well worth it

Lazydog

I have found the following table to help me with subnetting;

     128   64   32   16   8   4   2   1
     ----------------------------------
128 | /1  /9   /17  /25
192 | /2  /10  /18  /26
224 | /3  /11  /19  /27
240 | /4  /12  /20  /28
248 | /5  /13  /21  /29
252 | /6  /14  /22  /30
254 | /7  /15  /23  /31
255 | /8  /16  /24  /32
     1st  2nd  3rd  4th

Numbers across the top are bits
Numbers down the left hand side are mask numbers
Numbers in the middle are the slash mask numbers.
Numbers at the bottom are the octets that the slash refers to.

If you are given a subent of x.x.x.x/15 looking at the chart you know the mask is 255.254.0.0 and the subnets are going to be going by 2's

This is all figured out by locating the '/15' on the chart.
Then looking at the bottom to see which octet the slash is referring to '2nd'
Then looking to the left to se what the mask is, '254'
Then counting down from the top of the row that the '/15' is in until you get to '/15' to know how may bits from the left you must count.
In this case it is 7 so counting from 128 right you count 7 until you arrive at '2'.

So your mask is 255.254.0.0
Your networks are:
x.0.0.0
x.2.0.0
x.4.0.0
x.6.0.0
ect.....

Hope I explained this well enough for everyone.

instant000

Lazydog wrote: »
I have found the following table to help me with subnetting;
     128   64   32   16   8   4   2   1
     ----------------------------------
128 | /1  /9   /17  /25
192 | /2  /10  /18  /26
224 | /3  /11  /19  /27
240 | /4  /12  /20  /28
248 | /5  /13  /21  /29
252 | /6  /14  /22  /30
254 | /7  /15  /23  /31
255 | /8  /16  /24  /32
     1st  2nd  3rd  4th 
Numbers across the top are bits
Numbers down the left hand side are mask numbers
Numbers in the middle are the slash mask numbers.
Numbers at the bottom are the octets that the slash refers to.

If you are given a subent of x.x.x.x/15 looking at the chart you know the mask is 255.254.0.0 and the subnets are going to be going by 2's

This is all figured out by locating the '/15' on the chart.
Then looking at the bottom to see which octet the slash is referring to '2nd'
Then looking to the left to se what the mask is, '254'
Then counting down from the top of the row that the '/15' is in until you get to '/15' to know how may bits from the left you must count.
In this case it is 7 so counting from 128 right you count 7 until you arrive at '2'.

So your mask is 255.254.0.0
Your networks are:
x.0.0.0
x.2.0.0
x.4.0.0
x.6.0.0
ect.....

Hope I explained this well enough for everyone.

Yes, this is about the smallest chart that can be used as a tool for subnetting.

But, I do agree with a previous poster. I would still recommend that the person know how to do it in binary also ...

so, at least have a chart running from

128 64 32 16 8 4 2 1

So that the person would be able to manually add together within a given octet, to figure out the decimal, from the binary.
(EDIT: Just saw that your chart does include from 128 thru 1 .... LOL ... good tool, overall.)

So many methods to subnet, just figure it out for yourself, your method is nice.

I can't even remember if hex conversions are in CCNA material or not, but if so, be aware of how to manually convert from binary to hex.

After knowing how to do it manually, then you can go into making **** charts for it.

Of course, nothing is faster (or more reliable) than doing it in your head

I dare say, it'd be so simple to just memorize it and be done with it, LOL.

CodeBlox

If it works for you, do it. Subnetting was SUPPOSED to be one of the "hardest" parts of the exam but once nailed, it's extremely easy. When I took ICND1, I didn't even use the dry erase board at all. They may try and throw a broadcast address at you and ask you other things about the diagram, when really it wouldn't work because a host is assigned a broadcast address.

The way I subnet is around the concept of a multiplier. The place value of the least significant subnet bit is the multipler.

Example: 192.168.1.0 /27

in the subnet mask 255.255.255.224, take the special octet which is 1110 0000 in binary.

The place value of the least significant subnet bit is 32: 1110 0000 = 224

Now that you have your multipler, you can use that to find your subnet boundaries.

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

Thats just how I do it and it works quite well.