Subnetting is not Fun

Subnetting is not fun exactly you when only off by one number.
Setup OSPF
Example :
Lab_B
e0:192.168.10.49/29
s1:10.255.255.255.82/30
s0:10.255.255.9/30
So I decided just for the fun/self practice of it to subnet 192.168.10.49/29

/29 =255.255.255.xxxxx000 or 255.255.255.252
128+64+32++16+8=252 with an increment of 4

192.168.10.0_________192.168.10.3
192.168.10.4_________192.168.10.7
192.168.10.8_________192.168.10.9

So i check myself and the Calculator says wrong:
/29 =255.255.255.xxxx0000 or 255.255.255.248 with an increment of 8

192.168.10.0_________192.168.10.7
192.168.10.8_________192.168.10.15
192.168.10.16_________192.168.10.23

How did I go wrong here?

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Comments

instant000

THA_DOC wrote: »

Subnetting is not fun exactly you when only off by one number.
Setup OSPF
Example :
Lab_B
e0:192.168.10.49/29
s1:10.255.255.255.82/30
s0:10.255.255.9/30
So I decided just for the fun/self practice of it to subnet 192.168.10.49/29

/29 =255.255.255.xxxxx000 or 255.255.255.252
128+64+32++16+8=252 with an increment of 4
192.168.10.0_________192.168.10.3
192.168.10.4_________192.168.10.7
192.168.10.8_________192.168.10.9

So i check myself and the Calculator says wrong:
/29 =255.255.255.xxxx0000 or 255.255.255.248 with an increment of 8
192.168.10.0_________192.168.10.7
192.168.10.8_________192.168.10.15
192.168.10.16_________192.168.10.23

How did I go wrong here?

Remember this sequence:
(Note: These are all your possible subnet values)
0
128
192
224
240
248
252
254
255

bits of subnetting -- mask
0 - 0
1 - 128
2 - 192
3 - 224
4 - 240
5 - 248
6 - 252
7 - 254
8 - 255

notice:
/8 = 255.0.0.0
/9 = 255.128.0.0
/10= 255.192.0.0
/11 = 255.224.0.0
/12 = 255.240.0.0
/13 = 255.248.0.0
/14 = 255.252.0.0
/15 = 255.254.0.0
/16 = 255.255.0.0
/17 = 255.255.128.0
/18 - 255.255.192.0
/19 = 255.255.224.0
/20 = 255.255.240.0
/21 = 255.255.248.0
/22 = 255.255.252.0
/23 = 255.255.254.0
/24 = 255.255.255.0
/25 = 255.255.255.128
/26 = 255.255.255.192
/27 = 255.255.255.224
/28 = 255.255.255.240
/29 = 255.255.255.248
/30 = 255.255.255.252
/31 = 255.255.255.254
/32 = 255.255.255.255

See the pattern yet?

Hope this helps.

higherho

THA_DOC wrote: »

Subnetting is not fun exactly you when only off by one number.
Setup OSPF
Example :
Lab_B
e0:192.168.10.49/29
s1:10.255.255.255.82/30
s0:10.255.255.9/30
So I decided just for the fun/self practice of it to subnet 192.168.10.49/29

/29 =255.255.255.xxxxx000 or 255.255.255.252
128+64+32++16+8=252 with an increment of 4
192.168.10.0_________192.168.10.3
192.168.10.4_________192.168.10.7
192.168.10.8_________192.168.10.9

So i check myself and the Calculator says wrong:
/29 =255.255.255.xxxx0000 or 255.255.255.248 with an increment of 8
192.168.10.0_________192.168.10.7
192.168.10.8_________192.168.10.15
192.168.10.16_________192.168.10.23

How did I go wrong here?

Instant 000 is 100% when I looked at your problem the first thing I glanced at was the /29 (255.255.255.248, this 248 tells me that octet changes) and the 4th octet value which is 49.

In the 5 network bits (last octet in your subnet mask .248 ) can you fit the value of 49 in it? So your network bits in the last part of the subnet mask has the following values 128,64,32,16, 8. By looking at the first five values I can tell that adding 32 and 16 gives me 48. This means that 192.168.10.48 is my network and 192.168.10.49 is my first host.

How do you get your last host? Remember you have 3 host bits (4,2,1 = 7). You simply add those three bits to your subnet ID and that gives you .55 (Broadcast) and then you subtract 1 off that to get your last host which is 192.168.10.54

so now we have:

Network - 192.168.10.48
First host - 192.168.10.49
Last host - 192.168.10.54
Broadcast - 192.168.10.55

capitanuionut

THA_DOC wrote: »

Subnetting is not fun exactly you when only off by one number.
Setup OSPF
Example :
Lab_B
e0:192.168.10.49/29
s1:10.255.255.255.82/30
s0:10.255.255.9/30
So I decided just for the fun/self practice of it to subnet 192.168.10.49/29

/29 =255.255.255.xxxxx000 or 255.255.255.252
128+64+32++16+8=252 with an increment of 4
192.168.10.0_________192.168.10.3
192.168.10.4_________192.168.10.7
192.168.10.8_________192.168.10.9

So i check myself and the Calculator says wrong:
/29 =255.255.255.xxxx0000 or 255.255.255.248 with an increment of 8
192.168.10.0_________192.168.10.7
192.168.10.8_________192.168.10.15
192.168.10.16_________192.168.10.23

How did I go wrong here?

It's quite obvious... i think in this way...
1. First i look at the class of the address - it's a class C address - this means 24 bits network and 8 bits host

2. Then i look at the actual subnet /29 - it means i have 3 bits left for the host portion (32-29 = 3)... 2^3 means 8 host per subnet (group of eight)

3. Remove network and broadcast address results in 6 host addresses .49 - .54

Hope this helps!

THA_DOC

Thanks everyone I see where I went wrong:
First 128+64+32+16+8=248 or /29 = 255.255.255.248
then
Then i look at the actual subnet /29 - it means i have 3 bits left for the host portion (32-29 = 3)... 2^3 means 8 host per subnet (group of eight)

I think keep practicing these for another week or so, before moving to the next subject. Then incorporate it as progress.
Again thanks for clearing things up instant000, higherho, and captainuionut.

odysseyelite

It takes time and there are so many different ways to subnet. If you don't like one way then you might want to try another. I'm not a math person, and the exam is geared towards people doing well with math in the crunch of time. I did not want to memorize my powers of two so I go the route of 256- subnet number (256-248 =

8 hosts.

After a afternoon of just doing problems over and over it all clicked. I for one, will be making the subnet chart on the scrap paper before the exam with the cidr notation and available host and networks. Easier to do it once than over and over again during the exam.

EastDallas

This website teaches a very simple method for subnetting:
Subnetting Made Easy And Other Cisco Tidbits: Subnetting Made Easy - Critically Acclaimed!

It helped me a lot.

I'm with odysseyelite. Despite your abilities to do subnetting mathematically, learning to make a subnetting chart conserves time during the exam and reduces mathematical errors. You still need to know how to do the calculations, but in an exam environment the chart is invaluable.

I wasn't able to find a link to show how to build the one I was taught. When I get home and have time, I'll pull out my notes and add it to this thread. You don't have to memorize much, it's done mostly using addition and subtraction.

Vokse

If you can get a hold of CBT Nuggets video series, there's some great lessons and tricks on subnetting. Definitely made it easier for me.

EastDallas

Here's a link to the subnetting chart and instructions on how to build it.

I hope my instructions are clear.

Subnetting Chart xls Download

THA_DOC

Vokse wrote: »

If you can get a hold of CBT Nuggets video series, there's some great lessons and tricks on subnetting. Definitely made it easier for me.

Thanks Vokse, I have and I am using it. Yes it does make it simpler. I am also looking for extra labs or anything to give more practice. I read here on this forum and from the videos that subnetting should done in under a minute. Therefore, I am searching, no hunting for labs on subnetting to buildup my timing and give me good pratice before moving into other CCNA topics this week.

THA_DOC

Thanks I will look at it.

THA_DOC

EastDallas wrote: »

Here's a link to the subnetting chart and instructions on how to build it.

I hope my instructions are clear.

Subnetting Chart xls Download

Thanks I will take a look at it. I really wat to get Subnetting to the point that I can do it off the top of my head. That weekend chore.

Vokse

THA_DOC wrote: »

Thanks Vokse, I have and I am using it. Yes it does make it simpler. I am also looking for extra labs or anything to give more practice. I read here on this forum and from the videos that subnetting should done in under a minute. Therefore, I am searching, no hunting for labs on subnetting to buildup my timing and give me good pratice before moving into other CCNA topics this week.

subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online

This is what I used to get my speed up.

EastDallas

I don't disagree that you should know how to do it off of the top of your head, but a lot of people who fail the exam do so because they run out of time. Subnetting can easily be the most time consuming thing you do on the exam.

My $ .02.

higherho

THA_DOC wrote: »

Thanks I will take a look at it. I really wat to get Subnetting to the point that I can do it off the top of my head. That weekend chore.

I was always told that I should do at least 10 sub netting questions a day until I can memorize the bits / powers of 2 like the back of my hand. The only reason why you need to get it down pact like this is the time issue on the exam that EastDallas stated.

defaultroute

Hey guys I love this forum! You know, as a social community tutor I teach subnetting to my students and it always is a big issue. So they kind pushed me into writing an e-book on the subject.

Now I'm not here to sell you the thing, you can decide for yourself when you read the link. All I can say if all my guys have passed their CCNA with very high grades and they all ace the IP addressing sections

The membership gives you access the e-book (45 pages of great information) plus two online exams which you sit just like the real one you will do when you sit the ISND/CCNA.

I wish you all the best of luck

Oh by the way, because it's on techexams I'm offering a 25% discount - feel free to pass on this link to anyone you know - the normal price if $9.95.

Discounted Binary Math Untangled

odysseyelite

Vokse wrote: »

subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online

This is what I used to get my speed up.

I used that site as well. I did pratice problems for an entire afternoon. I still made a chart for the exam because I was worried with time and math errors. I could never do math in my head even in grade school.