VLSM Variable Length subnet masks

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VLSMVariable Length subnet masks

• Subnetting the subnet

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IP Address Design Example

• Company with 200 hosts
• Assigned class C network 195.10.1.0/24
• Want 6 different LANs
• Subnet class C network using mask 255.255.255.224
  (/27)
• Whole network has the same subnet mask or
  supports equal number of hosts on all the
  sub-networks
• Is this a problem?
• Why?
• What should be done to solve it ?

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VLSM (Variable Length Subnet masks)

• VLSM Subnets a subnet
• VLSM divides a network into subnets of various
  sizes to prevent wasting IP addresses
• Uses up wasted addresses
• Conserves address space on WAN links
• Point-to-point link only needs 2 addresses
• With VLSM point-to-point links can use a /30 mask

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VLSM

• Variable Length Subnet Masking allows division of
  address space based on the size of networks
• Start with network requiring the most addresses
• Create a subnet mask
• (use CIDR Classless Inter Domain Routing)
• Subnet the subnet as needed to provide address
  space required for other subnets
• Be logical start at beginning or end or address
  space
• Addresses must be contiguous to enable route
  summarization

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Task

• Question
• If an ISP has a 200.1.1.0 Network, he wants to
  give assign to 5 customers who need 20, 12, 6,
  25, 18 hosts.
• What should the subnet mask be?
• What is the address range in each subnet?
• What should the routing table entries be?
• Can it be done efficiently to conserve IP
  addresses?

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VLSM - example
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(No Transcript)
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CIDR Classless Inter-Domain Routing

• Address format ltIP address/prefix Pgt.
• The prefix denotes the upper P bits of the IP
  address.
• Can be used to specify arbitrary blocks of
  addresses
• Say an ISP has 192.5.48.0, 192.5.49.0,192.5.50.0,
  192.5.51.0 then
• the IP address advertised will be 192.5.48.0/22
• An ISP can obtain a block of addresses and
  partition this further to its customers
• Say an ISP has 200.8.4/24 address (256
  addresses)
• He has another customer who needs only 4
  addresses from 200.8.4.24 then that block can be
  specified as 200.8.4.24/30

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Task

•  

Given IP address 204.15.5.0/24, use VLSM to
create a subnetting scheme
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Additional Tasks

• Please try other questions on VLSM, available on
  module web page