Classful vs Classless Addressing
Last Updated : 26 May, 2025
Classful and Classless addressing are methods used in networking to manage IP addresses. Classful addressing divides IP addresses into fixed classes (A, B, C, D, E), each with predefined ranges. In contrast, classless addressing, also known as CIDR (Classless Inter-Domain Routing), offers more flexibility by allowing addresses to be subdivided into smaller blocks called subnets. This flexibility helps optimize address allocation and supports the growth of the internet by efficiently managing IP address resources.
Classful Addressing
Classful addressing was introduced in 1981, with classful routing, IPv4 addresses were divided into 5 classes(A to E), each with a predetermined range. The class of an IP address determines the network portion and the host portion based on its class-specific subnet mask. Classful addressing was inflexible and led to inefficiencies in address allocation, which prompted the development of classless addressing (CIDR) for more efficient use of IP address space.
- Classes A-C: unicast addresses
- Class D: multicast addresses
- Class E: reserved for future use
Classful AddressingClass 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. Its Subnet mask is 255.0.0.0.
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. Its Subnet mask is 255.255.0.0.
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. Its Subnet mask is 255.255.255.0.
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. Its Subnet mask is not defined.
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. Its Subnet mask is not defined.
- Class A with a mask of 255.0.0.0 can support 128 Network, 16,777,216 addresses per network and a total of 2,147,483,648 addresses.
- Class B with a mask of 255.255.0.0 can support 16,384 Network, 65,536 addresses per network and a total of 1,073,741,824 addresses.
- Class C with a mask of 255.255.255.0 can support 2,097,152 Network, 256 addresses per network and a total of 536,870,912 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 Addressing
Classless Addressing or Classless Inter-Domain Routing was introduced in 1993 to replace classful addressing. Classless Inter-Domain Routing (CIDR) is a method for efficiently allocating IP addresses and routing Internet Protocol (IP) packets. Unlike classful addressing, which divides IP addresses into fixed classes (A, B, C, etc.), CIDR allows for variable-length subnet masks (VLSM). This means that networks can be divided into smaller, more flexible subnets according to their specific needs, rather than being constrained by predefined class boundaries.
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 contiguous, 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.
Difference Between Classful Addressing and Classless Addressing
| Parameter | Classful Addressing | Classless Addressing |
|---|
| Basics | In Classful addressing IP addresses are allocated according to the classes- A to E. | Classless addressing came to replace the classful addressing and to handle the issue of allocation of IP Address. |
| Network ID and Host ID | The changes in the Network ID and Host ID depend on the class. | There is no such restriction of class in classless addressing. |
| VLSM | It does not support the Variable Length Subnet Mask (VLSM). | It supports the Variable Length Subnet Mask (VLSM). |
| Bandwidth | Classful addressing requires more bandwidth. As a result, it becomes slower and more expensive as compared to classless addressing. | It requires less bandwidth. Thus, fast and less expensive as compared to classful addressing. |
| CIDR | It does not support Classless Inter-Domain Routing (CIDR). | It supports Classless Inter-Domain Routing (CIDR). |
| Updates | Regular or periodic updates | Triggered Updates |
| Troubleshooting and Problem detection | Troubleshooting and problem detection are easy than classless addressing because of the division of network, host and subnet parts in the address. | It is not as easy compared to classful addressing. |
| Division of Address | | |
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