Difference between Classful Routing and Classless Routing

Routing Protocols help the routers to guide them in transferring the data from source to destination. Routing protocols are broadly classified based on their operation, behavior, and purpose. The purpose involves the gateway protocols and the operation deals with distance vector and link state routing protocol. Classful and Classless routing comes under the category of behavior. The primary difference between the classful and classless protocols is that the routing updates do not include subnet mask information and include subnet mask information respectively. In the current trend, modern networks do not use classful routing.

What is Classful Routing?

In routing updates, Classful Routing does not include the subnet mask and when a route update is sent the same subnet mask must be used by all connected devices. The subnet mask is nothing but the 32-bit number with all the bits as 0 on the host side and all 1’s on the network side. Some examples of classful routing protocols are RIP and IGRP.

RIPv1 and IGRP were the first two Internet Protocol version 4 (IPv4) routing protocols to be created. They emerged as a result of the allocation of network addresses based on classes A, B, and C. At that time the purpose of developing classful routing is that the first octet of the network address could be used to determine the network mask.

Example of Classful Routing

Let A and B be two routers and Router A does not include the subnet mask for carrying the routing update.

Case-1 − If the routers are directly connected, the host will use the subnet mask of the interface network.

Case-2 − If the routers are not directly connected, the routing difficulties occur and it assigns based on the order of classes.

What is Classless Routing?

During routing, it will send the subnet mask along with the routing update information. Variable Length Subnet Masking is used by classless routing. Some of the newer protocols that use this classless routing are RIP-2, OSPF, and EIGRP. Routing protocols using IPv6 are classless. Only IPv4 routing protocols normally fall under the classification of being classful or classless. Since all IPv6 routing protocols incorporate the prefix length with the IPv6 address, they are all regarded as classless.

On the other hand, classless routing protocols send an IP address along with a prefix length. As a result, classless routing protocols can aggregate networks into a single entry and identify those groups using the prefix length. Furthermore, unlike the classful scheme, which limited prefix lengths to 8, 16, or 24, classless routing protocols accept any prefix length.

Example of Classless Routing

Let A and B be routers, the Router A carries the route update information along with the subnet mask. The Router A interface receives the update when belonging to the same major network then Router B will recognize it after accepting the routing update.

Difference between Classful and Classless Routing

Classful and classless routing is one of the categories of routing protocol that also provides major characteristics while transferring data through the routers. The Classful is a type of traditional method and classless routing is used in the latest networks.

Basis of Difference

Classful Routing

Classless Routing


It does not forward subnet mask information and routing updates simultaneously.

It would forward subnet mask information and the routing update simultaneously.

Variable Length Subnet Masking (VLSM) subnet

VLSM is a subnet or a piece of a large network that is not supported in classful routing.

In classless routing, VLSM is supported during the routing update.

Hello message parameter

This routing does not use a “Hello message” for checking the status.

“Hello Status” is used to check the status of neighbors.


The Bandwidth consumption is very high in classful routing.

Classless routing consumes less bandwidth.


The Addresses are classified into three parts subnets, networks, and hosts.

The Addresses are classified into three parts subnets and hosts.

Routing updates

There are regular or periodic updates of the routing information.

Triggered updates (only partial) are used in classless routing.

Discontiguous network

It does not support a discontiguous network but the subnets are not visible to each other.

It supports a discontiguous network but the subnets are visible to each other.

Major Protocols and their network size

RIP-1 and IGRP have small network sizes.

The Network size of RIP-2 is small, while EIGRP and OSPF are larger.

Implementation and Maintenance

The cost of implementation and maintenance is low.

The cost of implementation and maintenance is high due to modern networks.

Error Detection

If any flaw occurs, it can be detected easily.

While in classless routing, error detection is tough.


The Routing protocols target to transmit data from one network to another or with the same network. Here the subnet which is a part of the large network plays a major role during update routing. Depending on the criteria for using the subnet masks, classful and classless routing protocols are used.

Updated on: 26-Apr-2023


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