QFT

Too true

What really matters in real life is getting it done properly, not quickly. It's not like it's going to take you a week

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www.mellowd.co.uk/ccie/

Subnetting rocked my arse when I took the ICND1. That subnetting workbook came in handy. You should definitely run through that book a couple times Scottsee. You can Google for the Teachers edition so you can check your work. But keep it only for that reason.

All in all, you just have to practice, practice, practice as I'm sure you know. Practice makes perfect. I really hate saying that though because subnetting practice can get boring and brain jello inflicting when you start dealing with crazy subnets.

I totally agree! And speed is only for the exam! At work or building networks U have enough time to figure out subnetting! So if U already good at it then it becomes butter!
lol

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LEARN WELL, PRACTICE, PRACTICE, PRACTICE!!

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It kind of is. I did this twice before my ICND1 and once before my ICND2. Practice makes perfect

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- Pete

Some sort of calculating math is going to be required until you have this down to a science.

I've figured out subnetting by taking the block size from the subnets CIDR and dividing the host part of the IP address by the block size. Then multiply the block size by the sum to get the subnet address.

If I need to figure out the subnet for 10.99.185.236/20
I know that a /20 is a block size of 16.
185 divided by 16 = 11.5625 or simply 11
16 x 11 = 176
The subnet address is 10.99.176.0
176 + 16 = 192
The broadcast address is 10.99.191.255 which is 1 address less than the next subnet 10.99.192.0.

I started writing out counting in the different block sizes and found common reference points to help hop along, like 10's. A block size of 8 if I'm looking for a subnet in the 200's 8x20=160 and 8x30=240. After that picking up the count is a little easier.

The valid hosts and subnet values can be easily figured by mastering your powers of 2 table.
A /20 is always going to have a 2 to the 12th calculating the host portion and a 2 to the 4th calculating the subnet portion (of course - 2). If you build a table that combines your CIDR, subnet mask, block size increment and powers of 2 values you'll engrain a cheat sheet in your head that can help you master subnetting in your head.

Some of the binary solutions were confusing to me and take way too long to convert. I am nowhere near as quick as I'd like to be, but even the best in the business use subnet calculators to figure out this stuff. When your troubleshooting a problem, you'd best have this down so you can get answers quick and be able to correct a mistake in addressing that someone else might make.

Besides resurrecting a thread almost a year old I don't think that I understand anything that you've posted
Seriously!
/20 means that the first 20 bits are the network portion of the address. That leaves 12 bits in the host portion of which up to 8 of the highest order can be used to increase the prefix to create subnets.
What's "block size" mean?
Easier for whom?
Totally baffling I'm afraid And you do not subtract 2 from the number of subnets, you subtract 2 from the number of addresses.
Not for me. I find the that doing the binary conversion is straightforward - why learn another complex method?

Hmm, ok, maybe I might get what your saying - if you assume a Class C network with a /20 then you have 4 subnet bits that gives you 16 subnets - is that what you mean by "block size"??

The only problem with that is default classful addressing disappeared from the CCNA in 2004/2005 and these days is just a historic relic from another era.

Aubrey

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The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn. Alvin Toffler, "Future Shock" 1970