VLSM and CIDR Variable Length Subnet Masking and Classless interdomain routing

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VLSM and CIDRVariable Length Subnet Masking and
Classless inter-domain routing
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VLSM and CIDR
This session will extend your understanding of
sub-networking and explore address
aggregation This session summarises pack 2 on
the OU T228 eDesktop site
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VLSM and CIDR

• Whilst IPv4 is 32 bit and offers over 4 billion
  addresses, severe restrictions now exist
• Companies may have public address space that is
  reaching saturation
• The loss of the broadcast subnet is considered
  wasteful
• VLSM (Variable Length Sub-Net Masking) has two
  advantages -
• It extends the hierarchical structure of the
  network addressing scheme
• Allows you to retrieve wasted addresses and apply
  them to suitable hosts

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VLSM and CIDR
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VLSM and CIDR

• The basic rules for VLSM are exactly the same as
  those for sub-networking,
• You can only borrow (subdivide) from the host
  portion of the subnet in question
• You can only have even numbers of variable
  sub-networks
• Binary 1s are always to the left, there are no
  exceptions for VLSM
• Binary 0s are always to the right, again there
  are no exceptions
• Binary 1s always represent the number of
  sub-networks
• Binary 0s always represent the number of hosts
  in each variable sub-network
• Borrowing is done with a 1, each 1 means that you
  double the number of sub-networks.
• The subnet mask is 32 bits, remember the dotted
  notation is only to make the information
  understandable to humans.

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VLSM and CIDR
An easy VLSM example! Create a subnet mask for
100.0.0.0 which will ensure it has 1500
sub-networks 100 .0 .0 .0 11111111 .11111111 .
11100000 .00000000 255 .255 .224 .0 It is
class A You must have 2048 sub-networks
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VLSM and CIDR

• An easy VLSM example!
• Create a subnet mask for 100.0.0.0 which will
  ensure it has 1500 sub-networks
• Now we want the 100th sub-network
• And yes we are going to use subnet 0 (TMA tip,
  declare it, use it)
• So we have the network id of 99 for the 100th
  sub-network (count from 0-99, is 100 networks)
• 99 in binary is 01100011
• Place it on the right hand side
• 100 .0 .0 .0
• 11111111 .11111111 .11100000 .00000000
• .00001100 .01100000 .00000000
• 100 .12 .96 .0
• Is our elected subnet

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VLSM and CIDR
An easy VLSM example! We have created a subnet
mask for 100.0.0.0 which will ensure it has 1500
sub-networks Now we want 100 sub-networks within
the 100th subnet So we simply start from our new
position 100 .0 .0 .0 100 .12 .96 .0 255 .2
55 .224 .0 11111111 .11111111 .11100000 .0000000
0 11111111 .11111111 .11111111 .11000000 255 .25
5 .255 .192 With 62 (64-2) hosts per subnetwork
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VLSM and CIDR
You must be aware, you can VLSM to the Nth degree
and VLSM a subnetwork that has already been
Variably Length Subnet Masked until you run out
of discrete addresses
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VLSM and CIDR

• VLSM problems
• With 99.0.0.0
• Create a subnet mask for 100 sub-networks
• Find the 50th (subnet 0 may be assumed)
• Create a subnet mask for a further 50
  sub-networks
• Now find the 25th
• Create a subnet mask for a further 10
  sub-networks
• With 200.0.0.0
• Create a subnet mask for 5 sub-networks
• What is the last sub-network?
• How many VLSM sub-networks can be fitted into
  this space

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VLSM and CIDR

• CIDR (Classless Inter Domain Routing)
• Commonly referred to as supernetting
• Is used to summarise a range of network addresses
  into one routing table entry
• Reduces the size of a routing table
• Reduces the updates on the inter router link
• Creation of one aggregate address
• Is not an exact science, which means we can have
  more than one answer to the same problem

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VLSM and CIDR
CIDR (Classless Inter Domain Routing)
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VLSM and CIDR
CIDR (Classless Inter Domain Routing)

• The process of supernetting is considerably
  simpler than sub-networking,
• as the desired outcome is to create an address
  and subnet mask
• which summarises a range of known (and previously
  sub-networked)
• network addresses.
• To calculate a supernet address, you have to -
• Line up all the network addresses
• Compare the binary
• Find the common bit!
• Calculate the new aggregate address and mask
  based on the common bit,
• and that which is to the left of it

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VLSM and CIDR
CIDR (Classless Inter Domain Routing)
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VLSM and CIDR
CIDR (Classless Inter Domain Routing) CIDR
Exercise For 80.16.10.0 80.16.20.0 80.16.30.0
80.16.40.0 80.16.50.0 Find a suitable summary
address
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VLSM and CIDR
CIDR (Classless Inter Domain Routing) CIDR
Exercise For 80.16.0.0 80.26.0.0 80.56.0.0 80.
66.0.0 80.76.0.0 Find a suitable summary address
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VLSM and CIDR
Additional Exercise?