Fast Subnetting Anyone?

I am in need a "robot-like" Fast subnetting trick(s). Plz-plz links.

Evey time I'm thrown a "hard" mask, my heart sinks, and I freeze up and take too long on doing subnets. I need a no-thinking, do-it robot-like method of passing the exams.

I don't need explanation of what subnetting do, why and all that, I know them well. I just need to be able to do them QUICK, 'cuz I have this mental block and try to analyze too much of what the exam is asking me.

TIA

Reply to
tester
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So, in short, you need a quick and easy way of doing what you find hard?

That comes with repetition and confidence, so just work at it!

172.14.x.x, and I want 48 hosts per segment

2 to the 6 - 2 = 62, 62>48>30, so 6 bits will give us enough hosts. We need

6 bits for the subnetwork. 32-6 = 26, so /26 is the prefix

16 bits for the class and network, 6 for the prefix, leaves 10 for the subnet

2 to the 10 - 2 = 1022 networks, 62 hosts per segment.

Double check using networks. First network is 172.14.0.64, and so on, 4 networks per 4th octet, 256 per 3rd octet, 4*256-2=1022 networks

The only "robotic" think I can think of that helped is knowing the powers of

2 cold.

-Owen

Reply to
Owen Roth

Try flash cards to memorize the CIDR to mask address and the powers of

  1. Then memorize the block number of the subnet mask i.e. 128=128
192=64 224=32 240=16 248=8 252=4 some call it the magic number. That number is the number between nets and one less is the broadcast. So a mask of /28 on a C address would start at the subnet number which would be XXX.XXX.XXX.0. The valid hosts are XXX.XXX.XXX.1 and goes to XXX.XXX.XXX.14 (subnet zero) the broadcast is 15. one less than the next net at 16. Next net starts at XXX.XXX.XXX.16 and the next at XXX.XXX.XXX.32, XXX.XXX.XXX.48 until you have 16 nets.
Reply to
aubuchonz

why 48?

i prefer 64

naarg just kidding mate :p you did very well, it's a matter of repetition heh, like in the old days, learning the tables of x ======================================================= NEW on the Internet: cisco irc chat groep irc://irc.intersecting.net/cisco.certification =======================================================

Reply to
polleke7

With enough repetition, memorization of these blocks is pretty automatic.

Next time out, I will write my lists before the clock starts: Power of

2 chart, bits-to-decimal chart (i.e. one bit = 128, two bits = 192 etc.), and prefix chart: /25 = 128, /26 = 192, /27=242, and so on.

I learned the hard way that speed is important. When you get an involved sim with multiple subnets to figure on question 52, you are sunk if you're not fast.

Reply to
greedo

This works for me: mask = 255.255.255.240

256-240=16 ( = block size, bocksize -2 is nr of hosts )

networks are : 16, 32, 48, 64 etc.... (repetetively add 16) broadcasts : 31, 47, 63, 79 (=next networknr -1) valid first hosts: 17, 33, 49, 65 (=network nr +1) calid last hosts : 30, 46, 62, 78 (=broadcast -1)

no powers, no inverting masks, just remember 256-mask..

Cas...

Reply to
Cas

i got the same prob man, class C i can handle, anyone should be able to handle it

class b i can kinda do it but i gotta use a pen and paper lol

aint tried a class A but i could probably do it, but it would take me years

like someone prev mkention powers of 2 and quick mental mathematical ability

i am like a donkey when it comes to maths, slow as can be! i am not crap, just slow

Reply to
Christo

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