Subnetting

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Subnetting
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Subnetting

• The network classes provide only two levels of
  hierarchy which isnt normally enough
• All of the network classes provide more than
  enough addresses for a single LAN
• An organization may have a Class B address but
  may need to subdivide it among several LANs
• Subnetting adds a layer to the hierarchy by
  breaking a single network address into separate
  smaller logical networks

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Binary to Decimal Conversion
1,024
1,000s 100s 10s 1s 1 0 2
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1010 1101
128s 64s 32s 16s 8s 4s 2s 1s 1 0
1 0 1 1 0 1
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Try it!
1010 1000 1001 1001 1111
1111
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Decimal To Binary Conversion
Convert 189 to binary
Choose a value (that is a place value in a byte)
that not higher than the value you are
converting. In this case the highest place
value, 128, is less than 189. Not subtract this
value from 189.
189 128 61 Gives a 1 in the 128 place
Now repeat the procedure with the 61. 64 is too
high which puts a 0 in the 64 place but 32 is
not too high so
61 32 29 Gives a 1 in the 32 place
29 16 13 Gives a 1 in the 16 place
13 8 5 Gives a 1 in the 8 place
5 4 1 Gives a 1 in the 4 place
1 1 0 Gives a 1 in the 1 place
End result is 1011 1101
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Try it!
75 161 223
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Masking

• Subnets are created using a technique called
  masking
• Masking is a technique used to extract partial
  strings of bits from a larger string of bits

Masking uses a logical AND operation in which 0s
are False and 1s are True 1 AND 1 1, 0
AND 1 False A byte 1010 0011 In this case
we want to extract the lower four
bits.
Mask 0000 1111 Result 0000 0011
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Default Subnet Masks

• The default subnet mask is used the separate the
  network portion of the address from the host
  portion
• These create no subnets
• Class A
• 255.0.0.0
• Class B
• 255.255.0.0
• Class C
• 255.255.255.0

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Default Subnet Masks
Dotted Decimal IP Address 130.57.1.56 Subnet
Mask 255.255.0.0 Binary IP Address 1000
0010.0011 1001.0000 0001.0011 1000 Mask
1111 1111.1111 1111.0000 0000.0000
0000 Network 1000 0010.0011 1001.0000
0000.0000 0000
Returns only the network portion of the address.
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Basic Subnetting Method

• Subnetting is accomplished by borrowing part of
  the node address to use as the subnet address

Example with Class B address 130.57.1.56 Subnet
Mask 255.255.255.0 Without subnetting
Example with subnettting Network address
130.57 Network address 130.57 Host address
1.56 Subnet address 1 Host address
56 Hosts 65,534 Hosts 254 Subnets
1 Subnets 254
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Subnet Masks
Dotted Decimal IP Address 130.57.1.56 Subnet
Mask 255.255.255.0 Binary IP Address 1000
0010.0011 1001.0000 0001.0011 1000 Mask
1111 1111.1111 1111.1111 1111.0000
0000 Network 1000 0010.0011 1001.0000
0001.0000 0000
The more bits used for the subnet address the
fewer hosts allowed.
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Subnets and Hosts
Max. subnets 2 (number of masked bits in subnet
portion of mask) - 2 Max. Nodes 2 (number of
unmasked bits in subnet portion of mask) - 2
Subnets 2 3 - 2 6 Nodes 2 13 - 2 8,190
Mask 255.255.224.0
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Procedure For Selecting a Subnet Mask

• Determine the maximum number of subnets or hosts
  you will need
• Determine how many bits you will need to borrow
  to give you the number of subnets you need
• Determine how many bits are left over for hosts
  and whether there are enough
• This will depend on the class of the network
  address

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Sample Problems

• Find the subnet mask assuming
• You currently have 25 offices with 100 hosts per
  LAN with a network address of 146.85.0.0
• You expect the number of offices and users to
  double in the near future

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Sample Problems

• Assume you have 5 offices with a maximum of 350
  hosts per subnet. Your network address is
  146.85.0.0. Which subnet mask will meet your
  requirements
• 255.292.0.0
• 255.255.252.0
• 255.255.192.0

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Sample Problems

• Find the subnet mask assuming
• You currently have 4 subnets and the largest has
  60 hosts per LAN with a network address of
  196.43.201.0
• You currently have 600 offices with a maximum of
  1000 hosts each. Your address is 10.0.0.0

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Sample Internetwork
Assume a network address of 65.0.0.0.