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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

Routing protocols

As we already know, the extended network prefix is used and therefore VLSM requires it to be carried out. To accomplish the VLSM requirement, two protocols are used OSPF and I-IS-I as the Interior Gateway Protocol, even though you could use RIP-2 a fixed in RFC 1388. These protocols carry the extended network prefix in each advertisement. If the protocol does not carry the prefix information, the router makes a lookup in its global configured routing table. If the information is not found there it applies a local mask and delivers the information as long as the host exists.

Make note that the higher the extended network prefix is, it has less hosts and gets faster to its destination. This is known as a forwarding algorithm.

For example if a /16 class B network has been defined as /22 bit extended network-prefix which has 64 subnets (26), that leaves 10 bits for the hosts which is equal to (210-2) 1022 hosts per subnet.

Now the same /16 Class B network has been defined as /26 extended network-prefix gives us 1024 subnets (210) “10 bits were required to have 1024 subnets”. That leaves only 6 bits for our hosts. Now (26-2) defines only 62 hosts per subnet.

In those two examples, if a packet is sent to the router, which network is evaluated first? The router obviously will pick the /26 extended network-prefix because it has the greatest bit number corresponding to the destination IP address; in other words it is a shorter route and is more descriptive.

To get a clearer picture observe the following, a host has been picked up for delivering a packet to 42.3.1.20

Route 142.0.0.0/800101010000000000000000000000000
Route 242.3.0.0/1600101010000000110000000000000000
Route 342.3.1.0/2400101010000000110000000100000000
Destination
Route3 => IP423120
Route342.3.1.2000101010000000110000000100010100
Table 8.12
|----------- /24 Network--------------|
|-------Host-----|

Note. In this picture there is a /8 and a /16 network; however, the router jumps and evaluates the /24 network first because it has a longer descriptive network prefix. According to what we have just seen, VLSM is the exact answer to using a network effectively. Now look at the following example. This is how a sub net is being sub-netted into more subnets.

Now let's assume that you were going to subnet 42.3.0.0/16 this address into other subnets; so let's do it. This IP address can be broken down as follows

Route 242.3.0.0/1600101010000000110000000000000000
Table 8.13

Before I convert this network into another network, note that this is currently a /16 network.

My intention is to create another 16 subnets from it, and since 24= 16; I need 4 bits more to create it, and by creating these subnets, I now convert my network into a /20 extended network prefix.

Route 342.3.0.0/200010101000000011
00000000
00000000
Table 8.14

12 bits are left for my host. (212-2) = 4094.
Now let's say that I need to break this further into a /27 extended network prefix. Now observe what happens.

Route 442.3.0.0/200010101000000011
00000000
00000000
Table 8.15

Since I had a /20 network already, I needed 7 more bits to make it a 27 Network prefix therefore (27) = 128, 5 bits are left for my hosts (25-2) = 30 So this is how it is done…
You could even break this further to restrict a network into smaller, less host support. (Ideal for small needs)