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  1. Agustin's Linux Manual
  2. Networks & Servers
  3. About the Author
  4. Table of Contents
  5. IP Addresses Networks and Subnets
  6. Network Classes
  7. IP Address in Decimal Notation
  8. Sub-netting
  9. Designing Subnets
  10. Allocating Subnets
  11. Defining Host Addresses
  12. Variable Length Subnet Mask
  13. Routing Protocols
  14. Classless Internet Domain Routing
  15. Servers - Chapter 9
  16. Apache Web Server
  17. Configuring Apache
  18. Uploading Web Pages
  19. Apache Overview
  20. MIMEMagic
  21. DNS Servers
  22. Welcome to Webmin
  23. Creating the Master Domain
  24. Adding the Reverse Zone
  25. Querying the DNS server
  26. Adding Virtual Domain to DNS Server
  27. Reverse Zone for Virtual Zone
  28. Binding IP Address for Virtual Domain
  29. Virtual Web Hosting
  30. DNS Security Options
  31. FTP Server
  32. Securing the FTP Server
  33. Email Server
  34. Postfix Configuration
  35. Dealing with Identical Users
  36. Configuring Email Clients
  37. Configuring Outlook
  38. Samba Server
  39. Configuring SAMBA Server
  40. The smb.conf File
  41. smb.conf Analysis
  42. Adding Users to Samba

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.

routing subnets
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.