Ok, I know that this topic has been beaten into ground, but this obviously my weakness. I am seeing references such as 192.168.1.0/24. I am assuming that /24 is the subnet mask, which is 255.255.255.0 because there are 8 bits in each octet. Down these same lines, something like 192.168.1.0/20 would be 255.255.240.0???? Also, in seeing something like:

Your network requires 10 subnets with 10 hosts per subnet, how in the hell do I equate this? I once learned this, but have completely forgotten how to do this! Please help!!!

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It comes down to understanding how and where to steal bits from. Check out www.learntosubnet.com. Some keywords you are searching for are supernetting and variable length subnetting.

bits: 1 2 4 8 16 32 64 128
nets: 2 4 8 16 32 64 128 256
hosts: 256 128 64 32 16 8 4 2
mask: 128 192 224 240 248 252 254 255

determining subnets: --->
determining hosts: <---

So if you have an IP of 192.168.2.2/25, you have to steal one bit to get from /24 to /25. This means a different subnet mask - look at the chart above and move one bit to the right - the new subnet mask (instead of 255.255.255.0) becomes 255.255.255.128.

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Last edited by Pavlov on 09-21-02 at 01:45 PM

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Thank you very much! Subnetting can be so confusing!!! In 7 years, I have never had to do it! Appreciate the help!

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I found a great subnetting guide:

http://www.certifyexpress.com/cisc/...g-cramnotes.pdf

I think this is the same guide that MCSEFREAK posted on his site.

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Posted this while back, hope it helps:

subnetting is pretty simple:
there 4 sets of 8 bits for masks:
00000000.00000000.00000000.00000000
The mask bits must be flip to 1 in sequence, that means you cannot do 11011000, it must 11111000. And every bit you move to the right. The bit weight starts from 128 and every bit that you move to the right, it halfs: 10000000 = 128, 110000000 = 128+64 = 192, 11100000 = 128+64+32 = 224... 11111110 = 128+64+32+16+8+4+2 = 254, you will not see 255.255.255.128 or 255.255.255.255.254 in subnetting a class C address as the two subnet are ilegal, because .128 does not provide any usable network number (N-2) and .254 does not provide usable hosts (H-2).

And the slash number is nothing but tells how many bits are masked. So, if /8 is the default class A, /16 is the default for class B and /24 is the default for class C.

Just know that if you want to calculate host, start the bits from the most RIGHT bit, then starts: 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 and minus 2 for usable host. If you are calculating networks, starts from the most LEFT bit: 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 and minus 2 for usable networks.

Example you are given a network of 192.168.5.0/24. You need max of 32 hosts, so, 2, 4, 8, 16, 32, 64 (you need 6 bits, as usable hosts are minus 2) that is 6 bits for hosts and from the right must be preserved as 0, that means you flip 2 bits for network that is 11000000 = 192 -> 255.255.255.192 or /26.

Example you are given a network of 172.13.0.0/16 and you need 1000 networks. So, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024, that is 10 bits from the left must be flipped to 1. So, since it is /16 so, we starts at 11111111.11111111.00000000.00000000 and flip 10 bits from the left: 11111111.11111111.11111111.11000000 and that is 255.255.255.192.

After you memorized the 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024, you pretty much do the subnet in your head.

Good Luck.

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Another good way to remember this is make a spreadsheet in excel that works it all out like above, and then you will be able to write it out on paper when you sit for the test.

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Yeah, I will do that. My test is Thursday. My nerves are already getting to me. I hate taking tests! I am such a mess before the tests. Appreciate the suggestion!

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Good luck with the test on Thursday Captain!

I've been trying to get my head round subnets, supernetting, etc. for the last week. I am like you, I learnt all this stuff a few years back and now it's totally forgotten!

I found http://www.mcsefreak.com/subnetting.htm really helpful!

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Appreciate it. Hope I get past this! I guess as they say, if you don't use, you lose it! Good luck to you!

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