Subnetting Tips - CCNA Power Camp, Ver. 1.00 Copyright 2005, Epsico 1 of 92

Subnetting Tips - CCNA Power Camp, Ver. 1.00 Copyright 2005, Epsico

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NetworkingNetworking TCP/IPTCP/IP

IP Addressing and Subnetting

Sandullah Epsicokhan


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Converting IP addresses from binary to decimal

• If you’re given a 32 bit binary number, break it into four groups of eight bits each.

Write

10010110100101100000001000000001

As

10010110 10010110 00000010 00000001


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Boolean AND operation

• The Boolean AND operation is a logical operation. It means exactly what it says. In order for the statement to be true we must have two things present.


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• For example if I said: “I have my right shoe and left shoe.” the statement would not be true if:

– If I only had my left shoe.

– If I only had my right shoe.– If I’m missing both shoes.


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• Similar logic is applied inside computers. Instead of shoes we use binary digits of 0 and 1.

• In computers the binary number 1 means we have something whereas the binary number 0 means we have nothing. Another way to look at it is that the binary number 1 means true in a binary number 0 means false.


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• Consider the following.

– 0 AND 0 = 0

– 0 AND 1 = 0

– 1 AND 0 = 0

– 1 AND 1 = 1


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Subnetting Practice

• AppA_CD.pdf on the Intro CD has 25 additional subnetting questions with answers.

• You can also check your results by using a subnet calculator.

http://www.subnet-calculator.com/



http://www.subnet-calculator.com/subnet.php?net_class=B


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Prefix notation

• Subnet masks made up of 1s and 0s. The 1s and 0s must be consecutive in a subnet mask. You cannot have 1s and 0s interspersed throughout the mask.

• The subnet mask always starts with 1s followed by 0s.


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Prefix location

• Prefix notation is simply a slash (/) followed by the number binary 1s in a mask. For example:– Binary 11111111 11111111 11111111 00000000

– Prefix notation /24

• We say the subnet has a 24 bit prefix or a slash 24 prefix.


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How many hosts, and how many subnets?

• Finding the sizes of the network, subnet, and host parts of the IP address.– The network part the address is always

defined by class rules.– The host part of the address is always

defined by the subnet mask. Binary 0s in the mask mean that the corresponding address bits are part of the host field.

– The subnet part of the address is what’s left over in the 32 bit address.


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Address 130.4.102.1

Mask 255.255.255.0

Number of network bits

?

Number of host bits ?

Number of subnet bits ?



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Address 130.4.102.1

Mask 255.255.255.0


16 (Class B)

Number of host bits 8 (Subnet mask has 8 zero bits)

Number of subnet bits 32 – (16 + 8) = 8

How many hosts, and how many subnets? NNNNNNNN . NNNNNNNN . NNNNNNNN. HHHHHHHH 8N 8N 8N 8H 24N + 8H


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Address 199.1.1.100

Mask 255.255.255.0


?





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Address 199.1.1.100

Mask 255.255.255.0


24 (Class C)



No subnetting.

How many hosts, and how many subnets? NNNNNNNN . NNNNNNNN . NNNNNNNN. HHHHHHHH 8N 8N 8N 8H 24N + 8H


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Understanding IP addressing

Address 130.4.102.1

Mask 255.255.252.0


16 (Class B)



NNNNNNNN . NNNNNNNN . SSSSSSS. HHHHHHHH 8N 8N 8S 8H 24N + 8H


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Understanding IP addressing

Address 199.1.1.100

Mask 255.255.255.224


?



NNNNNNNN . NNNNNNNN . NNNNNNNN. SSSSSSSH 8N 8N 8N 8H 24N + 7S + 1H


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Key methods

• Convert the mask binary, using any method for conversion at your disposal, and count the number of zeros.

• Convert the mask to binary after memorizing the nine decimal and binaries values and tables 12-8

• Create your own binary conversion table.



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Subnetting formulas

Number of subnets =

2(number-of-subnet bits)-2

Number of hosts per subnet =

2(number-of-hosts-bits)-2


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Create a decimal conversion table

• When working with binary numbers and octets, one way to do the math is the start with a decimal conversion table. Start by creating the following table.


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BitCounts 1 2 3 4 5 6 7 8

Table S-1 Number of bits in a byte


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Create a decimal conversion tableTable S-2 Value of each bit (working from right to left)

BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20


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BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1

Table S-3 Decimal Value of the powers of two. Each is twice as much as the bit to the left.


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Align the binary number to the table

BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1

BinaryNumber 1 0 0 1 0 1 1 0

Table S-4 Align the bits to the decimal values.


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Keep the decimal values where the number 1s appear Then add the results

BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1


Result 128 0 0 16 0 4 2 0

Sum 128 + 16 + 4 + 2 = 150

Table S-5 Multiple the binary value times the decimal value above it. Then add up the result.


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Exercise 1: Build the conversion tableExercise S-1 Build the conversion table


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Exercise 1: Build the conversion table

BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Exercise S-1 Lay out the bit positions


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Exercise 1: Build the conversion table

BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1

Exercise S-1 Double the values


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Exercise 2: Convert to decimal

00000000


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BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1


Result 0 0 0 0 0 0 0 0

Sum 0



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01010011


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BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1


Result 0 64 0 16 0 0 2 1

Sum 64 + 16 + 2 + 1 = 83



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11111111


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BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1


Result 128 64 32 16 8 4 2 1

Sum 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255




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10101101


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BitCounts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal

Value 128 64 32 16 8 4 2 1


Result 128 0 32 0 8 4 0 1

Sum 128 + 32 + 8 + 4 + 1 = 173




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Convert decimal to binary• If X is a number we wish to convert

from decimal to binary, perform the following steps building the binary number from right to left until X = 0.

1. If X is odd, write down the number “1” and subtract 1 from X.

2. If X is even, write down “0”. 3. Divide X by 2. 4. Go back to step 1 using the result from

step 3.


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Convert decimal to binary

•For example, let's take the decimal value 157:157 - 1 = 156, 156 ÷ 2 = 78 78 - 0 = 78, 78 ÷ 2 = 39 39 - 1 = 38, 38 ÷ 2 = 19 19 - 1 = 18, 18 ÷ 2 = 9 9 - 1 = 8, 8 ÷ 2 = 4 4 - 0 = 4, 4 ÷ 2 = 2

2 - 0 = 2, 2 ÷ 2 = 1 1 - 1 = 0, Result from bottom up

10011101


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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks

Build a subnetting tableTable S-5 Add current subnet value to current decimal value


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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0

128



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128

128

192



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192

128

192

224



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224

128

192

224

240



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224 + 240

128

192

224

240

248



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224 + 240 + 248

128

192

224

240

248

252



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224 + 240 + 248 + 252

128

192

224

240

248

252

254



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224 + 240 + 248 + 252 + 254

128

192

224

240

248

252

254

255



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Bit Counts 1 2 3 4 5 6 7 8

Power 27 26 25 24 23 22 21 20

Decimal Value 128 64 32 16 8 4 2 1

Masks + 0 + 128 + 192 + 224 + 240 + 248 + 252 + 254

128

192

224

240

248

252

254

255



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Questions?

Documents

Subnetting Tips - CCNA Power Camp, Ver. 1.00 Copyright 2005, Epsico 1 of 92