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Vlsm exercises solutions

Written By KAZIPUR Information Technology-KIT on Monday, December 28, 2015 | Monday, December 28, 2015



1. Data Networking Year 2 Variable length Subnet Masking 2. Why VSLM
  • Normal Subnetting forces use to choose one subnet mask for the whole network
  • Can be wasteful
  • e.g. Small sub offices will each get the small IP allocation as head office
  • Especially wasteful for WAN comms (where only two addresses needed)
3. VSLM
  • Answer is to allow multiple subnet masks to exist within the one network
  • Each physical segment can have it's own specific subnet mask given it an appropriate number of hosts
  • Normally uses the slash notation
  • VLSM can be considered hierarchical
    • Effectively subnetting a subnet
    • Important to think like this in design
    • So work down from Network->Subnet->Subnet etc.
4. VLSM vs CIDR
  • VLSM is effectively the same as CIDR but focusing inside an organisation
  • Same notation used
  • Same calculation concepts as CIDR
    • Network specified at bit level
    • Must fall on a valid boundary
    • Route summarisation can be done
5. VLSM Class B Example
  • Large Irish Republic company
  • Offices in 32 counties, large HQ in Dublin
  • 750 hosts in Dublin
  • 20 hosts in each county office
  • WAN links between each office
  • They have one Class B – 132.1.0.0
  • Will it do?
6. VLSM Class B Example
  • ~65,000 hosts available in a Class B
  • We need 750 + (31 * 20) + (32*2) hosts
    • i.e. Dublin + County Offices + 2 per WAN
    • 1434 hosts required
  • Normal subnetting forces us to accommodate the subnet with the largest number of hosts
  • 750 hosts => /22 subnet mask
  • This allows for 1022 hosts and 62 subnets
  • But we need a subnet for each WAN link
    • 32 + 32 for the WAN = 64
  • So it wouldn't fit
7. VLSM Class B Example
  • But with VLSM
  • 750 hosts implies the first tier – 62 /22 networks
    • 132.1.0.0 /22 (#0) (Not used, all o's)
    • 132.1.4.0 /22 (#1)
    • 132.1.8.0 /22 (#2)
    • ... 132.1.252.0 /22 (#64) (Not normally used, all 1's)
  • If we drop subnet zero then 132.1.4.0 /22 will be used for Dublin
8. VLSM Class B Example
  • Now need 31 subnets of 20 hosts
    • 20 hosts => /27 block
    • The are 32 /27 blocks in a /22 block
    • OK!
  • Also need 32 subnets of 2 hosts for WAN
    • Usually taken from the last block of the top hierarchy
    • 2 hosts => /30 (dropping all o's and 1's)
    • 32 /30's will easily fit in the last /22 without hitting the final 1's subnet
9. VLSM Class B Example
  • So we have the whole network covered
  • Still have 60 /22 blocks left – over 60,000 hosts
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