Variable Length Subnet Mask
There aren't many ways to really use a network efficiently. The RFC 1009, implemented in 1987; which states that it is possible to use more than one subnet due to the different sizes of the extended network prefixes.
As I said earlier having a flat network is a great disadvantage, because once the subnet mask is defined; it locks the network into a fixed number of subnets.
Other important aspects of VLSM are the further division of address space, by dividing an address into subnets, and then on to more subnets, which is known as aggregation.
Fig 8.9 Note how this network has been divided recursively.
Our base network is a /8 network, but then it is sub-netted into a /16 extended network prefix. Observe that in the second stage, the /16 network is sub-netted into two other subnets /24 networks and /19 extended network prefix. It doesn't stop there. Note that the /24 network was sub-netted into a /27 extended network prefix.
Designing networks of course needs a little of artistry…but don't expect an audience to be clapping at your performance. So far I haven't seen people paying for $500 dollar tickets to see an engineer performing on the most highly complex of engineering theory. I mean people make more money performing on a TV show than solving society's problems by developing communications systems.
Put your imagination to work. This is the way underground networks are created. Each subnet can have its own router and can be interconnected within other networks through gateways, proxies, etc.
Fig 8.10 routing subnets.
Observe the picture and note how VLSM can reduce the size of the routing table. First of all note that the entire sub-netted network is not visible to the outside world, and everything is going out trough router A. Depending on the size of the network, the bandwidth may be exhausted with all active connections, you could easily load balance this by treating any part of the network as a totally separated network and inserting another gateway.
Designing Variable Length Subnet Mask Networks
When designing networks, a lot of things are taken into consideration. For example, how many hosts are currently required, and how many would be most likely needed in the future. Many organizations have different locations that they want to include as part of the project.
- First note that variable Length goes by level; so consider your designs as levels (everything that will be needed).
- Always think today and future.