Classful Addressing

 In Classful Addressing System, IP Addresses are organized into following 5 classes-

 

 

1. Class A
2. Class B
3. Class C
4. Class D
5. Class E

 

1. Class A-

 

If the 32 bit binary address starts with a bit 0, then IP Address belongs to class A.

 

In class A IP Address,

• The first 8 bits are used for the Network ID.
• The remaining 24 bits are used for the Host ID.

 

 

Total Number Of IP Addresses-

 

Total number of IP Addresses available in class A

= Numbers possible due to remaining available 31 bits

= 2³¹

 

Total Number Of Networks-

 

Total number of networks available in class A

= Numbers possible due to remaining available 7 bits in the Net ID – 2

= 2⁷ – 2

= 126

(The reason of subtracting 2 is explained later.)

 

Total Number Of Hosts-

 

Total number of hosts that can be configured in class A

= Numbers possible due to available 24 bits in the Host ID – 2

= 2²⁴ – 2

(The reason of subtracting 2 is explained later.)

 

Range Of 1st Octet-

 

We have-

• Minimum value of 1st octet = 00000000 = 0
• Maximum value of 1st octet = 01111111 = 127

 

From here,

• Range of 1st octet = [0, 127]
• But 2 networks are reserved and unused.
• So, Range of 1st octet = [1, 126]

 

Use-

 

• Class A is used by organizations requiring very large size networks like NASA, Pentagon etc.

 

2. Class B-

 

If the 32 bit binary address starts with bits 10, then IP Address belongs to class B.

 

In class B IP Address,

• The first 16 bits are used for the Network ID.
• The remaining 16 bits are used for the Host ID.

 

 

Total Number Of IP Addresses-

 

Total number of IP Addresses available in class B

= Numbers possible due to remaining available 30 bits

= 2³⁰

 

Total Number Of Networks-

 

Total number of networks available in class B

= Numbers possible due to remaining available 14 bits in the Net ID

= 2¹⁴

 

Total Number Of Hosts-

 

Total number of hosts that can be configured in class B

= Numbers possible due to available 16 bits in the Host ID – 2

= 2¹⁶ – 2

 

Range Of 1st Octet-

 

We have-

• Minimum value of 1st octet = 10000000 = 128
• Maximum value of 1st octet = 10111111 = 191

 

So, Range of 1st octet = [128, 191]

 

Use-

 

• Class B is used by organizations requiring medium size networks like IRCTC, banks etc.

 

3. Class C-

 

If the 32 bit binary address starts with bits 110, then IP Address belongs to class C.

 

In class C IP Address,

• The first 24 bits are used for the Network ID.
• The remaining 8 bits are used for the Host ID.

 

 

Total Number Of IP Addresses-

 

Total number of IP Addresses available in class C

= Numbers possible due to remaining available 29 bits

= 2²⁹

 

Total Number Of Networks-

 

Total number of networks available in class C

= Numbers possible due to remaining available 21 bits in the Net ID

= 2²¹

 

Total Number Of Hosts-

 

Total number of hosts that can be configured in class C

= Numbers possible due to available 8 bits in the Host ID – 2

= 2⁸ – 2

 

Range Of 1st Octet-

 

We have-

• Minimum value of 1st octet = 11000000 = 192
• Maximum value of 1st octet = 110111111 = 223

 

So, Range of 1st octet = [192, 223]

 

Use-

 

• Class C is used by organizations requiring small to medium size networks.
• For example- engineering colleges, small universities, small offices etc.

 

4. Class D-

 

If the 32 bit binary address starts with bits 1110, then IP Address belongs to class D.

 

• Class D is not divided into Network ID and Host ID.

 

 

Total Number Of IP Addresses-

 

Total number of IP Addresses available in class D

= Numbers possible due to remaining available 28 bits

= 2²⁸

 

Range Of 1st Octet-

 

We have-

• Minimum value of 1st octet = 11100000 = 224
• Maximum value of 1st octet = 11101111 = 239

 

So, Range of 1st octet = [224, 239]

 

Use-

 

• Class D is reserved for multicasting.
• In multicasting, there is no need to extract host address from the IP Address.
• This is because data is not destined for a particular host.

 

5. Class E-

 

If the 32 bit binary address starts with bits 1111, then IP Address belongs to class E.

 

• Class E is not divided into Network ID and Host ID.

 

 

Total Number Of IP Addresses-

 

Total number of IP Addresses available in class E

= Numbers possible due to remaining available 28 bits

= 2²⁸

 

Range Of 1st Octet-

 

We have-

• Minimum value of 1st octet = 11110000 = 240
• Maximum value of 1st octet = 11111111 = 255

 

So, Range of 1st octet = [240, 255]

 

Use-

 

• Class E is reserved for future or experimental purposes.

 

Classes of IP Address-

 

All the classes of IP Address are summarized in the following table-

 

Class of IP Address	Total Number of IP Addresses	1st Octet Decimal Range	Number of Networks available	Hosts per network	Default Subnet Mask
Class A	231	1 – 126	27 – 2	224 – 2	255.0.0.0
Class B	230	128 – 191	214	216 – 2	255.255.0.0
Class C	229	192 – 223	221	28 – 2	255.255.255.0
Class D	228	224 – 239	Not defined	Not defined	Not defined
Class E	228	240 – 254	Not defined	Not defined	Not defined