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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

Classless Internet Domain Routing (CIDR)

Well this is what I call a lifesaver…Do you remember the problem with 32-bit IPV4? Classless Internet routing is what really solved the routing problem and was originally documented in 1993 RFC 1517, 1518, 1519, and 1520.

Because of the limitation of class A, B, and C, the IETF proposed the protocol to eliminate these barriers by efficiently allocating existing IPV4 addresses, and the result is IPV6.

But how were things actually fixed? IPv6 or CIDIR is very clever, it actually take advantage of these classes A, B, or C. Any given network prefix, will actually define the amount of IP addresses.

Let's say that we have a 42.3.0.0 /20

Route 342.3.0.0/200010101000000011
00000000
00000000
Table 8.16

So we have 12 bits left for our host correct? (212) = 4096 these are the actual corresponding hosts on each network, very cool isn't it? Look at the following block of networks

Class A10.25.40.0 /200000101000011001
00101000
00000000
Class B128.36.0.0 /201000000000100100
00000000
00000000
Class C192.168.1.0 /201100000010101000
00000001
00000000
Table 8.17

Even though CIDR can solve a lot of problems, it may cause you a lot of problems too… be informed of what type of networks you encounter, especially ISPs. Remember IPV6 is fairly new and it is still under deployment. And yes, CIDR uses the address space very efficiently.

Here is an example, let's say that you are an ISP and you have a block of 42.3.64.0 /18

Route x42.3.64.0/180010101000000011
01000000
00000000
Table 8.18

One of your clients just asked you for 1000 IP addresses. You could easily create a subnet /22 extended network prefix.

Route x42.3.64.0/220010101000000011
01000000
00000000
Table 8.19

So 4 bits were required to create the extended network prefix that is (24) = 16 subnets and leaves 10 bits for the hosts. Then this is (210) = 1024 hosts per subnet; the result is that you could easily give the client one of the 16 subnets.

That's it … Congratulations! Now you know about network engineering.