Define classful addressing and discuss problems associated with it.

 

A classful network is a network addressing architecture used on the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993. The method divides the IP address space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits. Classes A, B, and C provide unicast addresses for networks of three different network sizes. Class D is for multicast networking and the class E address range is reserved for future or experimental purposes.

 

Classful Addressing Definition

Under classful network addressing, the 32-bit IPv4 address space was partitioned into 5 classes (A-E) as shown in the following tables.

Classes

(Sources: https://media.geeksforgeeks.org/wp-content/cdn-uploads/gq/2015/07/nethostdata.jpg)

Bit-wise representation

In the following bit-wise representation,

·         n indicates a bit used for the network ID.

·         H indicates a bit used for the host ID.

·         X indicates a bit without a specified purpose.

Class A

  0.  0.  0.  0 = 00000000.00000000.00000000.00000000

127.255.255.255 = 01111111.11111111.11111111.11111111

                  0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH

 

Class B

128.  0.  0.  0 = 10000000.00000000.00000000.00000000

191.255.255.255 = 10111111.11111111.11111111.11111111

                  10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH

 

Class C

192.  0.  0.  0 = 11000000.00000000.00000000.00000000

223.255.255.255 = 11011111.11111111.11111111.11111111

                  110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH

 

Class D

224.  0.  0.  0 = 11100000.00000000.00000000.00000000

239.255.255.255 = 11101111.11111111.11111111.11111111

                  1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

 

Class E

240.  0.  0.  0 = 11110000.00000000.00000000.00000000

255.255.255.255 = 11111111.11111111.11111111.11111111

                  1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

The number of addresses usable for addressing specific hosts in each network is always 2N - 2, where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host value to represent the network address and the all-bits-one host value for use as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the maximum number of hosts are 254.

Today, IP addresses are associated with a subnet mask. This was not required in a classful network because the mask was implied by the address itself; Any network device would inspect the first few bits of the IP address to determine the class of the address and thus its netmask.

The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., 0.0.0.0/8 and 127.0.0.0/8 are reserved in former class A; 128.0.0.0/16 and 191.255.0.0/16 were reserved in former class B but are now available for assignment; 192.0.0.0/24 and 223.255.255.0/24 are reserved in former class C. While the 127.0.0.0/8 network is a Class A network, it is designated for loopback and cannot be assigned to a network.

Class D is reserved for multicast and cannot be used for regular unicast traffic. Class E is reserved and cannot be used on the public Internet. Many older routers will not accept using it in any context.

 

(Source: https://en.wikipedia.org/wiki/Classful_network#Classful_addressing_definition)

 

 

Problems with Classful Addressing:

The problem with this classful addressing method is that millions of class A addresses are wasted, many of the class B addresses are wasted, whereas, number of addresses available in class C is so small that it cannot cater to the needs of organizations. Class D addresses are used for multicast routing and are therefore available as a single block only. Class E addresses are reserved.

Since there are these problems, Classful networking was replaced by Classless Inter-Domain Routing (CIDR) in 1993. We will be discussing Classless addressing in the next post.

(Source: https://en.wikipedia.org/wiki/Classful_network)