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BlueWolf's Howl

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November 17, 2002

Handy Dandy

Doesn't it always seem that you end up learning things the hard way first, and then someone shows you a shortcut?

Subnetting was never easy. In fact, there are online tools that can figure out your subnetting for you. There are tables to memorize and a lot of bald admins because they pulled out all their hair while trying to subnet a network.

Subnetting divides a network address into a bunch of smaller networks. Admins do this to make management easier. Well, it's easier once you have the subnets and the ranges of valid addresses... Getting those addresses and figuring out the number of networks and hosts can be difficult.

I first learned about subnetting while studying for the TCP/IP exam for the MCSE. It was convoluted and difficult and I studied hard and was able to defeat the subnetting monster. One of the lucky things is that the Microsoft exams allowed you to use the calculator during the exam. You could convert decimal to binary and figure it all out from there. Then there was the Cisco exam... You're not allowed to use the calculator and have to figure it all out yourself.

I first took the CCNA (Cisco Certified Network Associate) exam about 3 years ago. Since it's been that long (gosh, how time flies), I now have to recertify on that exam. The first time I tackled it, I tried to memorize as much as I could of the subnetting stuff. I knew how to convert decimal to binary. I had to go as fast as I could through the regular questions to afford myself enough time to work out the subnetting questions. It was a lot of pressure, but I made it.

So as I'm reading chapter 3 for this exam...I run across a section devoted to subnetting in your head. Yes, subnetting in your head!!! I couldn't believe it. Oh shit, it must be a table to memorize. Put it off, put it off, put it off... Okay, so I put off studying the rest of the chapter for about a week. Now I'm looking at the calendar and grabbing myself by the scruff of the neck and making myself study again. Just plow through it and you'll eventually get to the other side...

I have seen the light and it is beautiful.

I can now subnet in my head thanks to Todd Lammle. As you may or may not know, there's a shortcut. I don't have to convert decimal to binary and all that other stuff. I just have to take the subnet mask and subtract it from 256 and it gives me the "blocks" for the various subnets. Then you add blocks to get up to the subnet mask. These are all your VALID (according to Cisco) subnets. [Note: for exam purposes, the zero network is not valid]

Example: host with a Subnet mask of
Find the subnet, broadcast address and valid network addresses

256-224 = 32 (these addresses are in blocks of 32)
32+32=64 subnet address broadcast address ---> range of valid addresses

Now all I have to do is memorize:
192 --- 224 --- 240 --- 248 --- 252
(128 and 254 are not valid)

OMG.... this is better than chocolate!

Posted by BlueWolf on November 17, 2002 07:30 PM