More about IP addressing- Classes of networks

Before we start about classes of networks following must be known:

Network address: 

The Network address uniquely identifies each network. It is a part of an IP address. Each machine on same network shares that network address as part of its IP address.

For example, in the IP address 172.10.40.66, 172.16 is the network address. (How ??, we are coming to that very soon.)

Host address: (or Node address)

Node/Host address is assigned to, and uniquely identifies each host/machine on a network. Unlike network address this can not be shared between different machines, this part of IP address must be unique for each machine on a network. IP address 172.10.40.66 represents 40.66 as the host address.

Lets begin with the Classes 

Classful addressing: 

Using the concept of classes for IP addressing is called classful addressing. Although this scheme is becoming out-dated, we must know briefly about it to discuss classless addressing.

In classful addressing address space is divided into five classes : A,B,C,D, and E.

(Note: Address Space is the total number of addresses used. )

	8 Bits	8 Bits	8 Bits	8 Bits
Class A	Network	Host	Host	Host
Class B	Network	Network	Host	Host
Class C	Network	Network	Network	Host
Class D	Multicast
Class E	Research

We can find the class of an IP address when given in both binary dotted decimal notation. Both of which are explained below, but, computer understands binary and to ensure efficient routing the designers made a policy for the leading bits of each class, the classes could be differentiated just by looking at the leading bits in binary notation of IP address.

Binary Notation

	First Byte	Second Byte	Third Byte	Fourth Byte
Class A	0
Class B	10
Class C	110
Class D	1110
Class E	1111

Decimal Notation

	First Byte	Second Byte	Third Byte	Fourth Byte
Class A	0-127
Class B	128-191
Class C	192-223
Class D	224-239
Class E	240-255

Practice question:
Find class of each IP address:
i) 00000001 00001011 00001010 00010101
ii) 11001010 00001011 11010100 00001010
iii) 172.16.10.1
iv) 192.168.1.0

Solution:
i) Class A    ii) Class C     iii) Class B    iv) Class C

Note: Although 0-127 represents Class A network but they are not used because of other purpose they serve which will be discussed later.

Now, one of the problem with classful addressing is  that each class is divided into fixed number of blocks with each block having a fixed size.
Number of Block = No. of possible networks for that particular class
Block size = No. of available hosts per network 
For an IP address, remember :
No. of possible hosts = 2ⁿ
where, n= host bits.
No. of possible Networks = 2ⁿ
where, n= Network bits.

Please note that, these are the figures of all possibilities. However all of them can not be used because some of them are reserved IP address

Classes	No. of Blocks	Block Size	Application
Class A	128	16,777,216	Unicast
Class B	16,384	65,536	Unicast
Class C	2,097,152	256	Unicast
Class D	1	268,435,456	Multicast
Class E	1	268,435,456	Reserved

Clearly, in class A the block is too large for any organisation. This means most of the addresses are wasted in class A. Similar problems were with the other classes.
So, In classful addressing a large part of the available addresses were wasted.


I guess, this is enough for IP addressing but before moving on to Subnetting.  
just a few more things to cover up Click Here