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How to subnet in easy steps – using a Happy chart – CISCO CCNA CCENT Subnetting

17/05/2015

Cisco certification questions will normally start with .. “you need 6 subnets” or “you need 25 hosts”, and they expect you to be able to calculate the correct subnets.  The answer to these certiification questions is to calculate a happy chart, when you first enter the exam room, as this will save you time during the exam.

Step 1 – Binary from 128 to 1 –  for the 8 bits

Each octet only has 8 bits.. so this is SIMPLE to do.

128   64   32  16  8   4   2  1

Step 2 – place a 1 above each binary value..

Subnetting is taking host bits and making them network bits.  So place a 1 above each of the binary numbers like this:

1      1     1    1    1    1   1  1

128   64   32  16  8   4   2  1

Step 3 – “ADD” the Mask to your happy chart

Basically this is adding up each set of binary numbers.  This is the “mask” that Cisco exams often give you.

ie 128 + 64 = 192.  Next column we go 192 + 32 = 224.  If you can add up – then the mask is a doddle to calculate.

1      1     1     1      1     1      1     1

128    64     32     16       8       4       2     1    (Basic Binary & the Magic number)

128  192  224  240  248 252 254 255  (MASK)

Step 5 – Calculate the subnets

Here’s the genius step.  Cisco exams will say “you need 2 subnets”.. so here you just put a row from left to right, starting at 2, and doubling in each column.  This tells you how many subnets you’ll get.

2     4     8    16    32                     (Number of subnets you want)

1      1     1     1      1     1      1     1

128   64   32     16      8    4      2     1    (Basic Binary & the Magic number)

128  192  224  240  248 252 254 255  (MASK – just add it up)

Step 6 – Calculate the number of hosts.

Sometimes the cisco exam will only say that you want 25 hosts.  Therefore you duplicate the doubling of the number 2 – but this time from Right to Left – to get a number of hosts required.  You can do this happy chart before the exam starts.. and then whizz through your exam.

2     4     8    16    32                     (Number of subnets you want)

1      1     1     1      1     1      1     1

128   64   32     16      8    4      2     1    (Basic Binary & the Magic number)

128  192  224  240  248 252 254 255  (MASK – just add it up!!)

*********************32    16     8    4     2  (number of hosts asked for in the exam)

If Cisco want 25 hosts, then you’ll have to go for 32 hosts.  The first ip is always the network id  The last ip is always the broadcast Id, ie 255.  “Usable” IP’s, are IP’s that can be assigned to hosts – this is the range of ip minus 2 Basically the rule is the number of Binary value less 2.  So 16 -2 = 14 usable IP’s.  32 -2 = 30 usable IP’s.  Easy right?

Step 7 – The magic number gives us the RANGE of a subnet.

When you put in the initial binary values, we called this the “magic number”.  This is your increment number – that you use to calcuate the ranges of each subnet.  This gives us the RANGE of IP’s in a subnet.

With a magic number of 64, the subnets will be:

a. b.c.0

a.b.c. 64

a.b.c 128

a.b.c. 192

You “jump” in increments of the magic number.  If you’re given a mask of /240 – then the happy chart says the magic number is 16 – so you’ll jump in increments of 16.

a.b.c.0

a. b.c 16

a. b. c. 32

Step 8 – the RANGE of IP’s.

Most Cisco exam questions will want you to work in a range of IP’s.

If the magic number is 16… then we’re jumping in 16’s.

Cisco want you to understand that within each RANGE..

Network IP = a.b.c.0

Broadcast IP = a. b. c. 15 (broadcast is always the LAST IP).

last usable ip = Broadcast Ip – 1  = a.b. c. 14

first uable ip = Network IP +1 = a. b. c. 1… (now, was that so hard to do?)

Remember your magic number will tell you the number to increment or JUMP in.  If the magic number is 64 – then the usable IP range is  64 – 2 = 62 usable IP’s. 

We increment in 64’s, from .0, .64, .128, .192.

a.b.c 0

a.b.c. 64

a.b.c 128

a.b.c. 192

The question will be phrased in several different ways, and they expect you to KNOW how to navigate these different angles of subnetting.

The key is to practice calculating the range of IP’s from the /26 or .240 mask, the number of subnets eg 4 subnets or the number of hosts eg 126 hosts – as that’s how cisco will examine you.

From → CISCO CCNA CCENT

One Comment
  1. Reblogged this on TheFlippinTruth.

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