VLSM Question

WillTech105

Hello all, I just took my ICND2 today but failed with a 798. I feel I lost points on the VLSM questions ( i got a good handle on the rest of the material).

According to subnettingquestions.com

Question: What is the first valid host on the subnetwork that the node 192.168.39.92/29 belongs to?

Answer: 192.168.39.89



Now my theory to the maddness is start at 192.168.39.92 and find the incrementing # -- which is 16. and count up

192.168.39.0
192.168.39.16
.32
.48
.64
.80 39.81 - 39.94 39.95 RANGE
.96

So I thought the asnwer was 192.168.39.81 but its 89.

I know there must be a better way than starting from 192.168.0.0 and counting ALL the way to 39. Am I missing something here?
Thanks in advance!

hiddenknight821

Right now I'm on a break between classes but I don't have time to reply to your post, so while you wait, you might want to check out this thread, given-address-identify-124th-subnet . I just helped out someone a while ago with VLSM and subnetting. Hope it helps.

wbosher

The increment for a /29 is 8, not 16.

hiddenknight821

WillTech105 wrote: »

Hello all, I just took my ICND2 today but failed with a 798. I feel I lost points on the VLSM questions ( i got a good handle on the rest of the material).

According to subnettingquestions.com

Question: What is the first valid host on the subnetwork that the node 192.168.39.92/29 belongs to?

Answer: 192.168.39.89



Now my theory to the maddness is start at 192.168.39.92 and find the incrementing # -- which is 16. and count up

I thought I didn't have enough time to answer, but I was wrong. I was able to spare a few minutes. You need to check your increment number again. It's always 2 + the number of hosts per subnet. You should figure this out.

To find the subnet id of the given host address, you should divide the octet (the one where there are 1s and 0s bits) by the increment number. You will get remainder. Ignore the remainder and multiple the whole number by the increment number. That should give you the subnet id. Again, I hope it helps. Apparently someone already beat me to it. By the way, this problem has nothing to do with VLSM at all.

WillTech105

Oops -- I see that my fault! That makes sense

192.168.39.88 39.89 - 39.94 with a b'cast of 192.168.39.95.

Now I was reminded as I was taking the test there is NO calculator. Are you suppose to count from 192.168.39.0 8+8+8+8 until you get to 80 - or is there a quicker way?


Another question from the site shows...
Question: Which subnet does host 172.28.57.172 255.255.255.240 belong to?
Answer: 172.28.57.160

Now I am thiniing 172.28.57.0 with an increment of 16.
172.28.57.0
.16
.32
.48
......
.160 172 is in this range. That makes sense.
.176

But even with a calculator I had to smahs the +16 button a good number of times. Like I mentione before, is there a quicker way to "jump" to the 100s instead of sitting in the testing room doing 16+16+16+16?

Jason0352

I usually start at .32/.64/.128 depending on where my range starts. Those numbers are always subnets when your increment is < 64

phobophile

I suppose it's just one of those things where you need to be pretty good and fast with doing math in your head.

In the above example, you can use a simple multiplication, like the number 10.

16x10 = 160
... (172) ...
160+16 = 176

Using another example:
Question: Which subnet does host 172.22.55.188 255.255.255.248 belong to?

The increment is 8 in the last octet.
The best way to look at it is in multiples of 5 or 10. In this case, I'll use the number 20.

8x20 = 160.

..168..
..176..
..184..
..( 188 )..
..192

So from there, you can see that the address belongs in the 172.22.55.184 subnet.

Hope that helps.

bermovick

I wrote down multiples of 16 during the 'pre-test' time I had, then worked within that.

32 increment is every other number written. For an 8 increment, just find the 16-increment that covers the block/ip/whatever I'm working in, then which 1/2 of the range I need = which 8-increment I need.

Jackace

wbosher wrote: »

The increment for a /29 is 8, not 16.

this^

0
8
16
24
32
40
48
56
64
72
80
88
96

so the subnet is 192.168.39.88 making your first valid host being 192.168.39.89 and the broadcast address for the subnet is 192.168.39.95

miller811

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

PhildoBaggins

memorizing and writing out a subnet chart from /17 through /32 across B and C will help out greatly in any test where subnet skills are required. You offload alot of brain horsepower since you simply look at the /19 CIDR and go "OH the increment is 8 so 0-7 8-15 16-31 etc....." and it will have xxx hosts and xxx total subnet (add 2 for zero ip sub rule)

erfolg255

Hi another way is do anding your ip address and mask by converting to binary

james0076ss

I use the cisco press method. by subtracting, 256 minus the non 0 or 255 #'ed octet in the mask, which gives you the increment. Than find out how many times the increment goes into subnetted part of the IP (which in the example below is 188

Question: Which subnet does host 172.22.55.188 255.255.255.248 belong to?
256-248=8
than
188 divided by 8 = 23 with the reminder of 4 (# your looking for)
than
188(subnetted #) - 4 (reminder) = 184

So the subnet host belongs to 172.22.55.184

This looks complicated but it's the fastest and best way I can come up with the answer

gosh1976

I use the method in this video but I've changed it just a bit to make it my own. https://learningnetwork.cisco.com/docs/DOC-3849

I stay away from binary conversions!! I write down the powers of 2 and the second chart that guy shows but I don't think my method is quite the same as his.

the chart looks something like this:

  8|  7|  6|  5|   4|  3|  2| 1     host bits
  1|  2|  3|  4|   5|  6|  7| 8     net bits
128|192|224|240| 248|252|254|255    dotted decimal notation
128| 64| 32| 16|   8|  4|  2| 1      Chunk size  (increment) 

some of you can probably get an idea of the method from just looking at the chart

PhildoBaggins

gosh1976 wrote: »

I use the method in this video but I've changed it just a bit to make it my own. https://learningnetwork.cisco.com/docs/DOC-3849

I stay away from binary conversions!! I write down the powers of 2 and the second chart that guy shows but I don't think my method is quite the same as his.

the chart looks something like this:

  8|  7|  6|  5|   4|  3|  2| 1     host bits
  1|  2|  3|  4|   5|  6|  7| 8     net bits
128|192|224|240| 248|252|254|255    dotted decimal notation
128| 64| 32| 16|   8|  4|  2| 1      Chunk size  (increment) 

some of you can probably get an idea of the method from just looking at the chart

I do this one, easy to memorize

INCR   128    64    32    16     8     4    2    1    128   64   32   16   8    4    2    1
CIDR   /17    18    19    20     21    22   23   24    25   26   27   28   29   30   31   32
MASK   128   192   224   240    248   252  254  255   128  192  224  240  248  252  254  255
HOSTS 32766 16382  8190  4094   2046  1022  510  254   126  62  30   14    6    2    0    0
Bsubn	0     2     6     14    30     62   128  254   510 1022 2046 4094 8190 16382 0     0                                      0    0
Csubn   1      1    1      1     1     1     1   1     0    2    6    14   30   62    0    0

Just wrote it out from memory to keep myself in check lol

WillTech105

Sorry for the slow reply -- I was away for a few days.

I appreciate everyone's input. I started making my own "chart" that is similar to the one above so I am planning for the next test (I have to wait until Nov 1) to write it all out so I don't waste time thinking of "so /13 is..." just look at the chart and bang.

Not that I am advocating charts. Once you learn the "under the hood" the chart just makes life alot easier -- and the test quicker. I still don't know why Cisco doesn't offer a calculator on the exam. It sure beats hitting 2+2+2+2+2+2+2+2 ect...

PhildoBaggins

WillTech105 wrote: »

Sorry for the slow reply -- I was away for a few days.

I appreciate everyone's input. I started making my own "chart" that is similar to the one above so I am planning for the next test (I have to wait until Nov 1) to write it all out so I don't waste time thinking of "so /13 is..." just look at the chart and bang.

Not that I am advocating charts. Once you learn the "under the hood" the chart just makes life alot easier -- and the test quicker. I still don't know why Cisco doesn't offer a calculator on the exam. It sure beats hitting 2+2+2+2+2+2+2+2 ect...

Cisco is one of those vendors who do not want their name tarnished by bad techs. I guess they figure if you can do it longhand or in your head then you are probably smart enough to get certified lol

I know pleanty of peeps that I call for a break/fix or a P1-P2 escalation that probably dont have access to a laptop for those utilties at 10pm on a saturday night. They can do alot of this stuff in there head, it makes them special lol

WillTech105

Yeah, I've heard that if you get a CCNA you should feel "special" since Cisco prides itself in making their tests harder than other vendors. But still -- its not like in the real world I can pull out my cell phone and open the Calculator app.

Anyway -- thanks again everyone. I'll keep at it and pass it come November 1st!