Routing - Classless and Classful routing protocols

Dynamic routing protocols such as EIGRP, OSPF, and RIP can behave in either a classful or classless manner. Classful and classless routing protocols differ in how they handle subnet masks.

Classful protocols, like RIPv1, do not include subnet masks in their routing updates, relying on default masks for class A, B, and C networks. In contrast, classless routing protocols, such as EIGRP, support the use of subnet masks or wildcard masks to specify more granular address spaces.

The only classful routing protocols are RIPv1 and IGRP, and these protocols are considered obsolete.

More modern classless routing protocols can be configured to behave in a classful manner. For example, with EIGRP, the auto-summary feature can achieve this. When auto-summary is enabled, EIGRP behaves classfully, summarizing routes to their classful boundaries. Disabling auto-summary (no auto-summary) allows EIGRP to operate in a classless manner.