V S L M

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V S L M

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

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