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)
- 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.
- 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
- 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?
- ~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
- 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
- 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
- So we have the whole network covered
- Still have 60 /22 blocks left – over 60,000 hosts