Subnets

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10.6-10.7

• Subnets

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? of possible hosts are there on a ?
3
Therefore, the Internet has

• 127 Class A licenses each with
  16,387,064 hosts
• 16,320 Class B licenses each with 64,516
  hosts
• 2,080,800 Class C licenses each with 254
  hosts
• What is the problem with this??

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The solutions to the problem are

• Multiple Class C licenses for companies(our
  school district owns about 15)
• Using Dynamic IP addressing
• Subnetting larger blocks of IP addresses(host
  IDs) into smaller blocks

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Subnetwork or Subnet

• Subnetworking- borrowing or stealing some of the
  bits of the host to create a new routing
  number(subnet)

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How do the devices know there is a subnet in the
address???
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Subnet Mask

• new number programmed into Layer 3
  devices(routers,etc.)
• 32 bits in length
• Put a binary 1 for the network portion
• Put a binary 0 for the host portion

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Class A11111111.00000000.00000000.000000000
255.
0.
0.
0
255.
0.
255.
0
255.
255.
255.
0

• Class B11111111.11111111.00000000.00000000

Class C11111111.11111111.11111111.00000000
This is with no subnetting
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Therefore with no subnetting
255.
0.
0.
0
255.
0.
255.
0
255.
255.
255.
0
The subnet mask will always be

• 255 in the network portion
• 0 in the host portion

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You Create Subnets !!!

• What if you have a Class A license and you dont
  want one big network with 16,387,064 hosts?
• Or
• You have a Class C network with only 254 hosts
  but you want to have smaller collision domains?

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How to create subnets?

• Use(borrow,steal,re-purpose) a part of the host
  portion of the IP address.
• Tell all routers that you are using part of the
  host portion for subnets by changing the subnet
  mask.

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Rules for subnetting

• Must use 2 or more bits and must leave at least 2
  bits in the host port
• Use the bits from the left.
• Add a binary 1 in the default subnet mask to
  create the new subnet mask.

Ex. Class A license with 2 bits of the host
portion borrowed for subnets. (network
address44.0.0.0)
Subnet mask changes from 255.0.0.0 to 255.192.0.0
or in binary
11111111.11000000.00000000.00000000
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Assignment

• Complete the subnet mask calculations for
  borrowing 2 or more bits from a
• Class A license
• Class B license
• Class C license
• Lets do some practice!!

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Ok, now How many subnets do you get?

• Figure out all of the new network(subnetwork
  numbers) you can get with new bits.
• Ignore the first subnet(reserved for network
  number)
• Ignore the last subnet(reserved for network
  broadcasts)

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Ex. Use 2 bits of the host portion to create
subnets

• 130.5.00??????.???????? 130.5.0.0
• 130.5.01??????.???????? 130.5.64.0
• 130.5.10??????.???????? 130.5.128.0
• 130.5.11??????.???????? 130.5.192.0
• therefore creates 4 total subnets, 2 useable

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• How many subnets would you get if you borrowed 3
  bits?
• 4 bits?, 8 bits?, 12 bits???
• Would you get the same number of subnets if you
  borrowed from a Class C license?
• Lets try a some subnetting!!

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Determining Subnetwork Numbers

• Use Boolean anding to determine the subnetwork
  number when given the IP address and subnet mask.
  This is what a router does!!!
• Ip 206.33.4.133 mask 255.255.255.192
• 11001110.00100001.00000100.10000101
• 11111111.11111111.11111111.11000000
• 11001110.00100001.00000100.10000000
• or subnet number 206.33.4.128

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Determining Subnetwork Numbers

• Use the formula 256/s where s the total number
  of subnets you will form
• ex. Borrowing 3 bits gives 8 total subnets,
  therefore 256/832. Subnet numbers are
• .0, .32, .64, .96, .128, .160, .192, .224

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Reserved IP addresses

• The first IP number in the network or subnet is
  reserved for the whole network. It cannot be
  assigned to a host.
• The last IP number in the network or subnetwork
  is always assigned for broadcast purposes.

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How many hosts do you get per subnet???

• You could figure out every binary combination of
  your remaining host portion
• Ignore the first host(reserved for network
  number)
• Ignore the last host(reserved for network
  broadcasts)

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Or you could use this handy-dandy formula!!

• 2h-2 where h the number of host bits remaining.
• Ex. Borrowing 3 bits from a Class C license
  leaves 5 bits for hosts. 25-2 32-2 30 usable
  hosts
• Lets practice ...

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What you should be able to do!

• Borrow bits to create subnets
• Determine of subnets
• Create subnet mask
• Identify subnet ID numbers
• Identify number of hosts/subnet
• Identify host range of numbers

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Alternatives to an IP Shortage

• DHCP(Dynamic Host Configuration Protocol)- share
  a pool of IP addresses among a group of
  computers. Ex. 380 computers in a building with
  one Class C license. Not all devices are on at
  the same time or are seeking network services.

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Alternatives to an IP Shortage

• Private IP Addressingex. Name Address
  Translation(NAT)- a range of IP addresses are
  reserved as private(not to be used on the
  internet). They include 10.x.x.x, 172.16.x.x and
  192.168.x.x. You can create your own LAN networks
  using these numbers with just one real IP
  connected to the internet. Also great for security

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Alternatives to an IP Shortage

• IPv6- there is some movement to shift all IP
  addresses to 6 octets of binary numbers. This may
  or may not happen due to the expense and hassle
  of new equipment, renumbering old systems, etc.
• Ex. 218.23.12.12.188.4