Classful Vs Classless Addressing

Classful Addressing:
Introduced in 1981, with classful routing, IP v4 addresses were divided into 5 classes(A to E).

Classes A-C: unicast addresses
Class D: multicast addresses
Class E: reserved for future use 

• Class A
  In a class A address, the first bit of the first octet is always ‘0’. Thus, class A addresses range from 0.0.0.0 to 127.255.255.255(as 01111111 in binary converts to 127 in decimal). The first 8 bits or the first octet denote the network portion and the rest 24 bits or the 3 octets belong to the host portion.

  Example: 10.1.1.1 

  Exception:

  - 127.X.X.X is reserved for loopback
  - 0.X.X.X is reserved for default network 

  Therefore, the actual range of class A addresses is: 1.0.0.0 to 126.255.255.255
• Class B
  In a class B address, the first octet would always start with ’10’. Thus, class B addresses range from 128.0.0.0 to 191.255.255.255. The first 16 bits or the first two octets denote the network portion and the remaining 16 bits or two octets belong to the host portion.

  Example: 172.16.1.1 

• Class C
  In a class C address, the first octet would always start with ‘110’. Thus, class C addresses range from 192.0.0.0 to 223.255.255.255. The first 24 bits or the first three octets denote the network portion and the rest 8 bits or the remaining one octet belong to the host portion.

  Example: 192.168.1.1 

• Class D
  Class D is used for multicast addressing and in a class D address the first octet would always start with ‘1110’. Thus, class D addresses range from 224.0.0.0 to 239.255.255.255.

  Example: 239.2.2.2 

  Class D addresses are used by routing protocols like OSPF, RIP, etc.
• Class E
  Class E addresses are reserved for research purposes and future use. The first octet in a class E address starts with ‘1111’. Thus, class E addresses range from 240.0.0.0 to 255.255.255.255.

Disadvantage of Classful Addressing:

1. Class A with a mask of 255.0.0.0 can support 16, 777, 214 addresses
2. Class B with a mask of 255.255.0.0 can support 65, 534 addresses
3. Class C with a mask of 255.255.255.0 can support 254 addresses

But what if someone requires 2000 addresses ?
One way to address this situation would be to provide the person with class B network. But that would result in a waste of so many addresses.
Another possible way is to provide multiple class C networks, but that too can cause a problem as there would be too many networks to handle.

To resolve problems like the one mentioned above CIDR was introduced.

Classless Inter-Domain Routing (CIDR):
CIDR or Class Inter-Domain Routing was introduced in 1993 to replace classless addressing. It allows the user to use VLSM or Variable Length Subnet Masks.

CIDR notation:
In CIDR subnet masks are denoted by /X. For example a subnet of 255.255.255.0 would be denoted by /24. To work a subnet mask in CIDR, we have to first convert each octet into its respective binary value. For example, if the subnet is of 255.255.255.0. then :

• First Octet:

  255 has 8 binary 1's when converted to binary 

• Second Octet:

  255 has 8 binary 1's when converted to binary 

• Third Octet:

  255 has 8 binary 1's when converted to binary 

• Fourth Octet:

  0 has 0 binary 1's when converted to binary 

Therefore, in total there are 24 binary 1’s, so the subnet mask is /24.

While creating a network in CIDR, a person has to make sure that the masks are contagious, i.e. a subnet mask like 10111111.X.X.X can’t exist.

With CIDR, we can create Variable Length Subnet Masks, leading to less wastage of IP addresses. It is not necessary that the divider between the network and the host portions is at an octet boundary. For example, in CIDR a subnet mask like 255.224.0.0 or 11111111.11100000.00000000.00000000 can exist.