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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

Allocating each of the 8 subnets

By creating 8 subnets, we basically created 8 different networks that can be allocated through different departments in the organization. These eight sub-networks will be numbered from 0 through 7.

Our source network is:

Base Net:	11000000. 10101000. 00000001. 00000000 = /24
Subnet #0:11000000101010000000000100000000=
Subnet #111000000101010000000000100100000=
Subnet #211000000101010000000000101000000=
Subnet #311000000101010000000000101100000=
Subnet #411000000101010000000000110000000=
Subnet #511000000101010000000000110100000=
Subnet #611000000101010000000000111000000=
Subnet #711000000101010000000000111100000=
Table 8.5

Rule of RFC 950

When RFC 950 was introduced, almost all routers were classful (RIP-1 Protocol); eventually classless (BGP-4 Protocol) routers were implemented. Because of the classful routers, RFC 950 prohibited the use of subnet 0s and subnet 1s.

But how does a router knows what is a 0s subnet? The answer is it doesn't. It is not an easy task for a classful router using RIP protocol, because RIP does not provide mask or prefix-length information. Therefore the router does not know which is a 0s subnet and which is the entire network. Here is why!

Subnet #0 route: /27 11000000. 10101000. 00000001. 000|00000
27-bit Prefix -->
Base network route: /24 11000000. 10101000. 00000001|. 00000000
24-bit Prefix -->
Table 8.6

As you can see, they look identical, but the true answer lies in the prefix length or mask. Refer to Figure 8.8. The solution to this 0's problem is the router's entry needed to be updated to include the prefix length in order to recognize which one was the 0's subnet and which one was the entire network.

For the subnet of 1s, the router requires the prefix length to determine whether the incoming package referred to broadcast of subnets 1's or to the broadcast of the entire network.

Broadcast to Subnet: /2711000000. 10101000. 00000001. 111|11111
27-bit Prefix -->
Broadcast to Network: /2411000000. 10101000. 00000001|. 11111111
24-bit Prefix -->
Table 8.7

You should also know that NET Builder software could automate solutions in relation to the 0s and 1s problem. This software allows the network administrator to Forward Subnet-Cast. Other solutions were implemented with the introduction of IGP (Internal Gateway Protocol).