Vlsm exercises solutions

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