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Chapter 3 - Subnetting and Class C Addresses

Cisco & IP Addressing
Louis D. Rossi, Louis R. Rossi and Thomas Rossi
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Chapter 3: Subnetting and Class C Addresses
Overview
  Tip All examples in this chapter assume a Class C address.
The best way to understand subnetting is to look at the bit pattern.
How many network addresses would be available with a 1-bit mask?
Table 3.1  All Combinations of a 1-bit Mask
128
64
32
16
8
4
2
1
Network
1-bit Mask
1
0
0
0
0
0
0
0
Combo 1
0
0
0
0
0
0
0
0
0
Combo 2
1
0
0
0
0
0
0
0
128
21–2=0
Combo 1 and combo 2 are illegal.
As a result networks 0 and 128 cannot be used and we have NO network addresses available.
Remember that we cannot use the all-zero or the all-one combination.
In later chapters we will continue our discussion of these two combinations and investigate times when we could use these addresses. These two combinations are called the zero and the broadcast subnet respectively.
How many network addresses would be available with a 2-bit mask?
Table 3.2  All Combinations of a 2-bit Mask
128
64
32
16
8
4
2
1
Network
2-bit Mask
1
1
0
0
0
0
0
0
Combo 1
0
0
0
0
0
0
0
0
0
Combo 2
0
1
0
0
0
0
0
0
64
Combo 3
1
0
0
0
0
0
0
0
128
Combo 4
1
1
0
0
0
0
0
0
192
22–2 =2
Combo 1 and combo 4 are illegal.
Networks that can be used are 64 and 128; as a result we have 2 available networks.
How many network addresses would be available with a 3-bit mask?
Table 3.3  All Combinations of a 3-bit Mask
128
64
32
16
8
4
2
1
Network
3-bit Mask
1
1
1
0
0
0
0
0
Combo 1
0
0
0
0
0
0
0
0
0
Combo 2
0
0
1
0
0
0
0
0
32
Combo 3
0
1
0
0
0
0
0
0
64
Combo 4
0
1
1
0
0
0
0
0
96
Combo 5
1
0
0
0
0
0
0
0
128
Combo 6
1
0
1
0
0
0
0
0
160
Combo 7
1
1
0
0
0
0
0
0
192
Combo 8
1
1
1
0
0
0
0
0
224
23–2 =6
Combo 1 and combo 8 are illegal.
Networks that can be used are 32, 64, 96, 128, 160, and 192. What do all these networks have in common, other than the fact that they are all even numbers?
How many network addresses would be available with a 4-bit mask?
Table 3.4  All Combinations of a 4-bit Mask
128
64
32
16
8
4
2
1
Network
Combo 1
0
0
0
0
0
0
0
0
0
Combo 2
0
0
0
1
0
0
0
0
16
Combo 3
0
0
1
0
0
0
0
0
32
Combo 4
0
0
1
1
0
0
0
0
48
Combo 5
0
1
0
0
0
0
0
0
64
Combo 6
0
1
0
1
0
0
0
0
80
Combo 7
0
1
1
0
0
0
0
0
96
Combo 8
0
1
1
1
0
0
0
0
112
Combo 9
1
0
0
0
0
0
0
0
128
Combo 10
1
0
0
1
0
0
0
0
144
Combo 11
1
0
1
0
0
0
0
0
160
Combo 12
1
0
1
1
0
0
0
0
176
Combo 13
1
1
0
0
0
0
0
0
192
Combo 14
1
1
0
1
0
0
0
0
208
Combo 15
1
1
1
0
0
0
0
0
224
Combo 16
1
1
1
1
0
0
0
0
240
24–2 =14
Combo 1 and combo 16 are illegal.
Networks that can be used are 32, 64, 96, 128, 160, and 192. What do all these networks have in common, other than the fact that they are all even numbers? Do you see a pattern developing?
The Multiple Process
In Chapter 1 we discussed the logical AND process to calculate network addresses. I would now like to introduce the multiple process.
With the multiple process instead of the logical AND process, it is easier to calculate the wire and broadcast address.
  Tip Network addresses are always multiples of the least significant subnet bit.
In the 3-bit mask example above, all the network address are multiples of 32.
Example 1
If we had an address of 192.16.12.52 with a mask of 255.255.255.240, we could easily calculate the wire address.
The 240 mask is a 4-bit mask, therefore all wire addresses are multiples of 16 (refer to Table 3.4).
To determine the wire address, answer the following question.
What is the highest multiple of 16 that is less than 52?
Since 48 is the highest multiple of 16 that is less than 52:
The network address is 192.16.12.48.
To determine the broadcast address, answer the following question.
What address is one less than the next wire address?
The next wire address is 64 (the next multiple of 16), so the broadcast address is 192.16.12.63.
Example 2
What is the wire and broadcast address of 192.30.20.17 with a mask of 255.255.255.252?
What is the highest multiple of 4 that is less than 17?
The network address is 192.30.20.16.
To determine the broadcast address, answer the following question.
What address is one less than the next wire address?
The next wire address is 20 (the next multiple of 4), so the broadcast address is 192.30.20.19.
  Note Just as a review of the logical AND process, examine examples 3.1 and 3.2.
Example 3.1
  Tip Keep in mind that the definition of a broadcast address is when all the host bits have a value of 1.
Example 3.2

 


 
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