Basic Subnetting

1
Basic Subnetting

• Computing Networks and Hosts
• Arnie Jordan

2
Basic Subnetting Part 1

• There are several ways to find the specifications
  of a network.
• Here is one simple way to identify the makeup of
  a network.
• Part I concentrates on finding the number of
  networks vs. the number of hosts.

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Basic Subnetting

• Five basic tasks in determining the number of
  networks and hosts
• Determine how many hosts or networks are needed.
• Identify how many bits were borrowed.
• Find the multiplier or magic number.
• Map the network using the multiplier.
• Find the subnet mask.

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Basic Subnetting

• Computing the number of networks is based on the
  binary equivalents of the following numbers
• 128 64 32 16 8 4 2 1

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Finding NetworksExample 1

• Given 192.168.11. ___
• Find a minimum of 12 networks
• 128 64 32 16 8 4 2 1
• Count right to left
• 1 2 4 8 1 14 networks

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Find Borrowed Bits the Magic Number

• Determine how many bits were borrowed 4
• 128 64 32 16 8 4 2 1
• Count the same number of digits left to right and
  draw a line.
• 128 64 32 16 8 4 2 1
• The magic number is left of the line 16

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Map the Network

• 192.168.11.16
• 192.168.11.32
• 192.168.11.48
• 192.168.11.64
• 192.168.11.80
• 192.168.11.96
• 192.168.11.112

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Map the Network

• 16 Network Address
• 192.168.11.16
• 31 Broadcast
  Address
• 32 Network
  Address
• 192.168.11.32
• 47 Broadcast
  Address
• 192.168.11.48

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Determine the Mask

• The mask allows the network to see only the
  network address.
• 128 64 32 16 8 4 2 1
• Add all the numbers left to right, including the
  multiplier 240
• Class C Mask is 255.255.255.240

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Another Example

• 202.151.37. ____
• What class?
• Break points 0 126 A
• 128 191 B
• 192 223 C

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Finding Networks

• 202.151.37. ____
• Need 2 Networks
• 128 64 32 16 8 4 2 1
• 3 networks
• 2 borrowed bits

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Find the Multiplier

• 2 bits were borrowed
• 128 64 32 16 8 4 2 1
• Magic Number 64

13
Map the Network

• 64 Network Address
• 202.151.37.64
• 127
  Broadcast Address
• 202.151.37.128
• 202.151.37.192
• 26 mask bits 6 host bits 32 bits
• Max subnets 3
• Max Hosts 62

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Compute the Subnet Mask

• Subnet mask 128 64 192
• 255.255.255.192

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Basic Subnetting

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You Try It

• 198.53.67._____
• With 6 bits borrowed
• Magic number ____
• Map the first three subnets with net broadcast
  addresses
• Subnet Mask ______