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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

How the extended network prefixes work

Well, the subnet mask is in charge of this. The subnet mask looks for the network prefix to ensure that a synchronized job is being processed. For example, if you need to specify a subnet mask in a class network, any subnet that you will specify will define your network

Extended Network Prefix
Fig 8.7

Even though the subnet mask is in charge of the lookup, the router protocol refers to the extended network prefix length rather than the actual subnet mask. Therefore the prefix length is contiguous 1bit in the subnet mask and because there is no standard which defines it, the routing protocol takes place to carry out the subnet mask. If the protocol fails to identify the subnet mask, the network just wouldn't exist, and all current process would be terminated.

Network Engineering, Designing Subnets

Example of Sub-netting:
If a company has been assigned a network number 192.168.1.0/24 and it needs to define six subnets; it is required to support at least 28 hosts for one of their departments.

16 (24), 8 (23), 4 (22), 2 (21)

First we need to determine the number of bits required for the six subnets.

  • Observe the power notation above, it is impossible to define six, therefore 23= 8 (3-bits are required). We have 2 unused subnets.
  • We are subnetting a /24 bit network prefix, our calculation result, was 3 bits required to define the 8 subnets; then 24+3 = 27 this makes our network an extended 27-bit prefix.
Subnet and Host numbers
Fig 8.8

Note. An IP address is a set of 4 octets of 8 bits.

  • This leaves us 5 bits for our hosts in each subnet. If we do the calculation for the five bits 25 gives us 32.

You may be wondering how we got that 224 subnet mask. The subnet mask is based on the 3-bits result of our 8 sub networks. Check the following table.

Subnet Expansion
Bit PatternMasked BitsProvided SubnetsSubnet Mask
11000000 222192
11100000 336224
11110000 4414240
11111000 5530248
11111100 6662252
11111110 77126254
11111111 88254255
Table 8.2

Yes, I know what you are saying, “The table doesn't tell me anything”. Look, this is how it goes:

Bit value1286432168421
Subnet
mask
- 192224240248252254255
#of
SubNework
- 26143062126254
Table 8.3

Here we go again, 3 bits was required for our 8 subnets, now look here:

Table 8.4
128 + 64 = 192 ----Number of Sub-networks
192 + 32 = 224 If 192 Subnet mask is used it
224 + 16 = 240 will give 2 subnets.
240 + 8 = 248  
248 + 4 = 252 If 224 Subnet mask is used it
252 + 2 = 254 will give 6 subnets.
254 + 1 = 255