CIDR Memorization Question page 120

[TKelley]

On page 21 and 22 of CCNA study guide fifth edition, the author talks about memorizing the CIDR notation for /26,/27,/28,/29 and /30.

Are these the only ones I should memorize?
What about the rest of them?

thanks,

Tim

[Katalysator]

Memorizing the notation? As in /27 > 255.255.255.224 ?

You should be able to convert any CIDR notation to a mask, and quickly.

If this IS what you're talking about, what I did is make sure I knew at LEAST the first four bits in an octet, i.e. :

255.255.255 = 24 bits

27 - 24 = 3

111 = 224 ( 128 64 32 )

You want to be able to easily remember:

1 128
11 192
111 224
1111 240
11111 248

You get the idea. When you remember that, its easy to do the 27 -24 = 3, 3 = 224 math in your head, and you can use your time more wisely.

Trust me, time really matters, and I mean every second.

[aguilera]

Time is key! forget about counting fingers and toes and thinking 1s and 0s before you even attempt to take the exam you need to have subnetting down!

Remember your simple math! Write it down before you start your exam! There will be no subetting calculator available to you, you cant bring in your own calculator or abacus or big chief notebook with a fat pencil! You just have to know it!

[Discover Cisco]

Memorizing the notation? As in /27 > 255.255.255.224 ?

You should be able to convert any CIDR notation to a mask, and quickly.

If this IS what you're talking about, what I did is make sure I knew at LEAST the first four bits in an octet, i.e. :

255.255.255 = 24 bits

27 - 24 = 3

111 = 224 ( 128 64 32 )

You want to be able to easily remember:

1 128
11 192
111 224
1111 240
11111 248

You get the idea. When you remember that, its easy to do the 27 -24 = 3, 3 = 224 math in your head, and you can use your time more wisely.

Trust me, time really matters, and I mean every second.
Thanks for the great advice.

I plan on studying the VLSM chapter soon.

@aguilera: so we're actually allowed to right down stuff on a piece of paper and use paper before the exam and use that same paper as a reference during the exam?

I've heard people say that before, but I'm not sure, I haven't seen anything about this in the exam policies.

[mr88talent]

You cannot bring any notes with you. Call potential exam centers before you register for the test and ask them if they provide paper and pencil as an alternative to the dry erase marker. If they don't - try another test center. The center I called said I could use 3 pieces of paper and a pencil - but only if I asked for them. You cannot write on the paper until the actual test starts however. As soon as the test starts - quickly write out your notes on subnetting. The multiples of 32 and 16 is a good table to have. You don't have a lot of time so make it short and sweet. Practice and plan what notes you intend on writing down well before the test. It sounds weird that you should memorize your notes, because if they are memorized you shouldn't need notes, but when faced with the stress of the test you don't want your brain to freeze up trying to remember that /28 is .240 with block size of 16.

I used something like this (properly lined up, that is). This gives me clues to both class C and class B subnetting:

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256

128 64 32 16 8 4 2 1
/25 /26 /27 /28 /29 /30
/17 /18 /19 /20 /21 /22 /23 /24
.128 .192 .224 .240 .248 .252 .254 .255


If you need to work with a block size of 8 (for example), it's obviously not hard to "work between the lines" and add 8 to a number above.