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Home > Archive > 70-216 > November 2003 > I'm feeling really stupid - subnetting
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I'm feeling really stupid - subnetting
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| carraway 2003-10-21, 12:31 pm |
| Can someone please explain subnetting w/ #s of subnets and hosts, etc... I just can't seem to get it. I have drawn the chart, read the resource kit on IP, but it's just not sinking in. This is my next to last exam, and this is really frustrating me!!!!  
Thanks. | |
| curiousgeorge 2003-10-21, 1:39 pm |
| I hope all of that helps. This is some of the info I used when I was an MCSE Instructor.
If you have one network ID (i.e. 160.20.0.0) but you want to set up your network so that it has multiple subnets, you must divide your network ID into smaller network ID’s. This is called subnetting. The way you subnet a network ID is by manipulating the subnet mask.
The equation used to find the appropriate subnet mask:
2^n – 2 => x
There are two variables, x and n. These variables have two combinations.
When
x = # of subnets needed
n = # of 1’s to add to your default subnet mask
When
x = # of hosts needed
n = # of 0’s in your subnet mask
You will be given x in the equation; you need to solve for n.
This is a reference chart to help solve the equation:
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
First, define x.
Do I need x number of subnets or do I need x number of hosts (computers) in each subnet?
Once you have defined x, put it in your equation and solve for n.
After you have solved for n in your equation, convert your default subnet mask into binary:
If x was the number of subnets needed, add 1’s to your default subnet mask from left to right
If x was the number of hosts per subnet, place 0’s in your subnet mask from right to left. After you place the required number of 0’s according to what you solved for n, the rest of the subnet mask will be 1’s
Once you have added the appropriate 1’s or 0’s to your subnet mask, convert back to a standard IP address.
This will be the subnet mask used for all of your subnets.
Subnetting Example:
You are given an IP address of 160.20.0.0.
You require 18 subnets for your network.
What is the proper subnet mask you must use?
First x = 18
Solve your equation:
2^n – 2 => x
x=18, therefore n=5
because
x= # of subnets required,
n= # of 1’s to add to your default mask.
This is a Class B address.
Your default subnet mask is 255.255.0.0
Converting your default subnet mask into binary:
255.255.0.0
11111111.11111111.00000000.00000000
Add 5 1’s to your default mask to get:
11111111.11111111.11111000.00000000
Converting this back to a normal IP address gives us a subnet mask of
255.255.248.0 | |
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| carraway 2003-10-21, 4:48 pm |
| curiousgeorge,
How is it that
x = 18
2^n – 2 => x
x=18, therefore n=5
because my calculation as well as your chart, 2^5 = 32, so 32-2 would = 30. | |
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| Powers of 2.
If I need 18 subnets, then let's see....(look at it in terms of closest number to 18 while providing for at least 18)
Let's try n=4:
2^4 - 2 = 16 - 2 = 14 subnets (hmmmm, that's not enough)
Try next one n=5:
2^5 - 2 = 32 - 2 = 30 subnets (Bingo, so we currently only use 18 out of the possible 30 subnets, no biggie. Save them for a rainy day, or when you need to expand your network  )
So, with a value of n=5, we get 30 subnets to use, and 30 is the closest number to 18 while providing for at least 18.
Get it, or did I confuse you more. | |
| curiousgeorge 2003-10-22, 12:01 am |
| Thanks for the assist Spid.
I forgot to include a small detail in my explanation (oops):
When solving the equation for n, use the smallest solution possible (i.e. if n=5 works in the equation and n=6, 7, 8, 9,... also works, use 5 because it's the smallest solution). And the equation is greater than or equal to- so it won't always be exactly what you want/need, but that's ok. | |
| karlisi 2003-10-22, 12:50 am |
| Whow! Thanks, curiousgeorge! At last one clear explanation of this. | |
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| carraway 2003-10-23, 9:59 am |
| Okay guys, after a few days of studying it, and reviewing it and realizing that I was making it WAY too hard, I finally got it down. Funny how you can work with this stuff everyday for 8+ years, and stumble over something like this.  | |
| stevenkok 2003-10-25, 4:54 pm |
| i lost the understanding for a year but by reading this iget it ALL back!!
Thanks guys. | |
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| Mossy 2003-10-29, 10:47 am |
| best book on subnetting I found was Sybex 70:216 NIA book. It has an entire chapter with great descriptions and 16 practice questions at the end of it. Also, the practice tests on learn to subnet were ok, if a little repetative. | |
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| ssgadams 2003-11-16, 12:38 pm |
| Great explaination guys!
I am prepare now to take the test ASAP not sure on subnetting part been awhile!
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| I found on both times I sat 216 that I only got 2 questions directly relating to subnetting. Good luck! | |
| carraway 2003-11-19, 2:43 pm |
| Hey guys,
Just an FYI, I just finished 70-220 Desgning Security. So, I'm done with my MCSE for Win2K! It's off to 292 and 296 for the udpate. Thanks for everyone's help!
Jason |
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