Subnetting Questions - Back from the dead

dazerski

Hello...

I am back after a long hiatus from CCENT studies. I was doing so well, actually understanding material, I built and practiced with a decent CCENT lab off of Ebay, I had studied all but 2 chapters in the Odom book, Jeremy's CBTNugget videos...everything... then I had subnetting left to master and I quit.

I didn't get it and when I was told that I had better be able to do those problems under 15 seconds... I thought that there was absolutely no way I'll ever get this test.

A few days ago, for the heck of it typed in Subnetting in YouTube and there was this guy with an english accent explaining how to subnet mask /26 in 1 minute. I watched it and was like, "No freakin way..." it appeared way too easy. So I watched it again. And again... and then tried a few problems on subnettingquestions.com and I got them all right. I about slapped myself all over my cube at work. I couldn't believe how easy it was. I vaguely remember doing it in school but it never stuck with me.

That video really sparked a fire in me to get this CCENT done because Subnetting was really the biggest road block and now I see a way through that road block. I just need to get faster at it. Some of the questions I can get through fairly fast using that "magic sheet" or whatever... but questions like this take me some time and I was wondering if anyone had any tips and tricks I could use to speed it up.


So if a question was... 156.88.133.104 / 255.255.252.0

Find the Network ID, 1st host, Last host and BC


Instead of going alllllll the way down, charting out

156.88.0.0
156.88.4.0
156.88.8.0
156.88.12.0
156.88.16.0 etc etc etc.... until I find what network 133 resides in, is there a faster way to do it? Granted that it isn't hard to count by 4's but I am looking to shave off any seconds that I can.


Any help would be appreciated. I know there are a gazillion threads out there that explain subnetting in various ways... I also wanted to start a thread just to kinda say hey again and be back on the board. Everyone helped me tremendously through the initial parts of my studies.


take care,
Dale

dave0212

This is general mathematics and an understanding of common numbers

I find the easiest way is to find the closest subnet that is common from the binary calc i.e 128,192,224 etc then it is much easier to work out

156.88.133.104/22 (255.255.252.0) as you pointed out gives increments of 4

So take 128 and work from there. next is 132 then 136 there is your network x.x.132.0
You know that the next network is 136 so x.x.135.255 is the broadcast anything after this is an easy calculation

This is how I do them, maths has always been a strong skill so I trained to the point I could look at an address and mask and work out the Network and Broadcast in about 10 seconds

Hope this helps in someway

stealthtt

The method easiest for me to understand is to AND the IP and Subnet mask in my head.

133=10000101
252=11111100

---

AND
132=10000100

128+64..too high
128+32..still too high
128+16..nope..
128+8..still no
128+4=132. (i stop here because I know the last 2 bits in the 252 mask are 0's, so they wont match.)

So we've got 132.0 for the Network.

For the broadcast, theres 2 0's in the .252 mask, so 2^2=4-1=3, 132+3 = 135.255


Thats how the thought process goes in my head.

dynamik

Yep, binary AND is the easiest. I have another example here: http://techexams.net/forums/viewtopic.php?p=248510#248510

Also, if you want to find the broadcast, invert the subnet mask (change 0s to 1s and vice versa), and do a binary OR with that address and the IP address you were given. Essentially, that just turns all bits to 1 that aren't 1 in the subnet mask. That might be an easier way of looking at it.

Edit: here's a link: http://www.shunsoft.net/ipcalc/help/chap07.html

Bokeh

Thats Paul from howtonetwork.net . His videos are very good, easy to understand. He is a nice guy as well, Ive exchanged several emails with him.

dazerski

Holy.... that's Awesome! THank you so much for the help and the others who posted too... that was a huge help as always; whenever I come here, I get support. That makes it so much faster than the long drawn-out way I was doing it.

thanks!

Dale

stealthtt wrote:

The method easiest for me to understand is to AND the IP and Subnet mask in my head.

133=10000101
252=11111100

---

AND
132=10000100

128+64..too high
128+32..still too high
128+16..nope..
128+8..still no
128+4=132. (i stop here because I know the last 2 bits in the 252 mask are 0's, so they wont match.)

So we've got 132.0 for the Network.

For the broadcast, theres 2 0's in the .252 mask, so 2^2=4-1=3, 132+3 = 135.255


Thats how the thought process goes in my head.

kevin31

Hi ddzierzek,

Im in the same boat as you stopped my studies as I hit subnetting and just cant seem to crack it. I can sit there and work out all the conversions but takes around 10 minutes which is far to long.

Id be greatful if you could point me in the direction of that video please? I have tried alsorts CBT nuggets, Trainsignal all the books you can buy but still no joy.

Hope you have now resumed your studying?

thanks

kev

dynamik

What are you having problems with? It seems like you're using good resources.

There's always www.subnettingquestions.com and www.learntosubnet.com if you want to try something else, but I'm not sure if they're going to give you any new information.

Kaminsky

ddzierzek wrote:

Find the Network ID, 1st host, Last host and BC


Instead of going alllllll the way down, charting out

156.88.0.0
156.88.4.0
156.88.8.0
156.88.12.0
156.88.16.0 etc etc etc.... until I find what network 133 resides in, is there a faster way to do it?

Firstly, welcome back

Whats wrong with finding (by standard mental arithmetic) which is the nearest 4 ?
IE 120 is divisible by 4 (theres 30 of them) leaving 13 remainder (obviously odd numnber so go 12) so it's 120 + 12 = 132.

so 156.88.132.0 is the subnet. Add .1 for the first host.

Then a nice quick way of finding the rest is just to add 4 again to 132 then go minues 1

156.88.135.255 for broadcast and 156.88.135.254 for last host. Every .255 in between is a valid host ip.

2 is easier, 8's and 16's can be a bit of a bugger but the same principal applies. from 32 onwards, it's dead simple. This is standard mental quick arithmetic and your brain is used to doing it wheras your brain is not used to doing boolean ANDs.
Good thing about boolean is that it's based around multiples of 2.


Subnetting is knowing boolean math and some rules about subnetting. Getting fast at it is just practicing your mental arithmetic.

Now working out number of subnets/hosts is trickier and I think best to memorise a table for that one.

dazerski

Kaminsky wrote:

ddzierzek wrote:

Find the Network ID, 1st host, Last host and BC


Instead of going alllllll the way down, charting out

156.88.0.0
156.88.4.0
156.88.8.0
156.88.12.0
156.88.16.0 etc etc etc.... until I find what network 133 resides in, is there a faster way to do it?

Firstly, welcome back

Whats wrong with finding (by standard mental arithmetic) which is the nearest 4 ?
IE 120 is divisible by 4 (theres 30 of them) leaving 13 remainder (obviously odd numnber so go 12) so it's 120 + 12 = 132.

so 156.88.132.0 is the subnet. Add .1 for the first host.

Then a nice quick way of finding the rest is just to add 4 again to 132 then go minues 1

156.88.135.255 for broadcast and 156.88.135.254 for last host. Every .255 in between is a valid host ip.

2 is easier, 8's and 16's can be a bit of a bugger but the same principal applies. from 32 onwards, it's dead simple. This is standard mental quick arithmetic and your brain is used to doing it wheras your brain is not used to doing boolean ANDs.
Good thing about boolean is that it's based around multiples of 2.


Subnetting is knowing boolean math and some rules about subnetting. Getting fast at it is just practicing your mental arithmetic.

Now working out number of subnets/hosts is trickier and I think best to memorise a table for that one.

Hi! I actually went and tried that AND'ing method and I guess it's the way numbers work in my head that I found it slower. I started using that other person's suggestion and getting close to higher numbers by multiplying by 10 and maybe dividing by 5 or something... just to move higher up faster. It's working out better for me. AND'ing on paper is much easier but in my head, it's easier and faster to use normal base-10 math. Yeah, 16's trip me up. Maybe I should ask my 12 year old brother to help me with my math LOL

And thank you! It's good to be back

dazerski

kevin31 wrote:

Hi ddzierzek,

Im in the same boat as you stopped my studies as I hit subnetting and just cant seem to crack it. I can sit there and work out all the conversions but takes around 10 minutes which is far to long.

Id be greatful if you could point me in the direction of that video please? I have tried alsorts CBT Nuggets, Trainsignal all the books you can buy but still no joy.

Hope you have now resumed your studying?

thanks

kev

This is the way I saw it on youtube http://www.youtube.com/watch?v=C4n2Ofqr6QY

I remember in school they taught us by doing this long drawn out convoluted method and since those days, I just assumed that was the way it was done and didn't know about so many shortcuts. This video helped me realize yet another way.

Definitely resumed my studies, and i mean hardcore. 4 hours after work, an hour before work, 30 minutes on my lunch and any other time I can squeeze in. I'm getting this cert on 9/30 and theres nothing else I'm gonna be doing with my time! I even canceled World of Warcraft... lol