Data Networking Year 2

Variable length Subnet Masking

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)

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.

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

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?

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

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

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

VLSM Class B Example

So we have the whole network covered

Still have 60 /22 blocks left over 60,000 hosts