My CCNA reference sheet


I am reading an older edition Todd Lammle’s CCNA book. I’ve been networking for seven years, but never got around to getting my CCNA, and sure enough, doing subnet related activities in the “fake world without calculators” drives me up the wall.

However, as I’m reading through the subnetting chapters 3, 4 and 5, I’ve written down some magic rules that I will memorize and immediately write down on a piece of paper when I arrive at the test, because we’re all living in a post-apocalyptic world without GPS, cellphones, the internet and, of course, calculators. (I wonder if the proctor at Pearson Vue will think it’s weird if I make my own ink out of carpet material, hair and plant matter and use my fingernail as a pen).

This won’t help you learn anything, because it’s all in context of theories. Sorry.

Base-2 maths things:

2^0 = 1   = 0000 0001
2^1 = 2   = 0000 0010
2^2 = 4   = 0000 0100
2^3 = 8   = 0000 1000

2^4 = 16  = 0001 0000
2^5 = 32  = 0010 0000
2^6 = 64  = 0100 0000
2^7 = 128 = 1000 0000

All bit-mask to decimal between /8 to /30 (ooo patterny):

_octet row_      1   2   3   4
0000 0000    0              /8
1000 0000  128 /25 /17 /9
1100 0000  192 /26 /18 /10
1110 0000  224 /27 /19 /11
1111 0000  240 /28 /20 /12
1111 1000  248 /29 /21 /13
1111 1100  252 /30 /22 /14
1111 1110  254 /31 /23 /15
1111 1111  255 /32 /24 /16

** `ip subnet-zero` allows for 255 and 0 length subnets.

Magic tip for the maths:

Write a table that contains 256 and subtract every 2^1-2^8 for quick reference while answering questions.

Subnetting rules:

Answer each question every time a subnet question is asked (unless it's quite obvious you don't need to).

How many subnets = 2^(number of network bits in the first "interesting" mask octet)
How many hosts per subnet = 2^(number of host bits) - 2
Valid subnets = 256 - (subnet mask bit decimal value given in the first mask octet) = (count increment for subnets).  If second or third mask octet present, treat it the same within its octet.
Valid network address = lowest subnet number
Valid broadcast address = highest subnet number

Not having enough fun? You have to practice subnetting like a maniac until you breeze through it.

VLSM block sizes:

VLSM block sizes:
Host: /31, /32
Class C: /24 through /30
Class B: /16 through /30
Class A: /8 through /30

Route summarization/aggregation/supernetting rules:

Use the highest order bit: Convert each subnet to binary, then see which binary bits are "in common."  "In common bits" is the mask in CIDR notation (add up the number of "in common bits", like 8+8+1+0 = /17).
"In common bits" in "the interesting" octet are the actual octet decimal, like 1000 0000 = 128, like n.n.128.0.

P.S. Still suck at subnetting Class A addresses.

P.P.S.: I hate VLSM, but maybe I just hate learning it, because of course, it makes sense; but maybe it’s too obvious. I mean, really: Given N, then you can just split it up and not be a moron (be efficient, minimize your expected advertised routes). Why all this stupid stuff with tables and charts?

P.P.P.S: Still hate the word “supernetting.”

IOS interface tips
Magic key combos to navigate around and functions.

  1. robb
    April 5, 2015 at 5:34 am

    There are some good but hard to solve question i have found here http://www.packettracerlab.com/Subnetting-Questions-IP-Subnetting.html

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