Classless Versus Classful IP Routing Protocols
The concepts of classless and classful IP routing protocols have roots in the manner in which IP
addresses originally were defined.
Under classful addressing rules, a network number was assumed to retain its natural mask unless
explicitly specified when subnetted into smaller blocks. However, earlier-generation routing protocols,
such as the Routing Information Protocol (RIP), could handle only a single mask for any address
throughout a network domain—the natural mask or a single consistent subnet mask. Routing
protocols such as RIP that cannot handle more than one type of mask, as in the case of VLSMs, are
referred to as classful protocols (see Table 1-3). The reason that classful protocols do not support
VLSMs is that, by design, they do not advertise or carry the associated subnet mask with routes and,
therefore, use simple intuitive mechanisms to determine the mask associated with a learned route.
The significant growth of the Internet to global dimensions called for more efficient use of the limited
IPv4 address space. Available addresses in the IP address space therefore attained the status of a
scarce commodity. The classless notions of VLSM and CIDR, discussed earlier, were invented to make
address allocation and use more efficient. Routing protocols also were enhanced to support classless
addressing environments. Routing protocols that are designed for operation in classless environments
and that can handle VLSM address and CIDR are referred to as classless routing protocols.
Table 1-3 features a list of routing protocols categorized as classful and classless. RIP-1 and IGRP are
grouped under classful protocols, whereas the more recently developed RIP-2, EIGRP, OSPF, IS-IS,
and BGP fall in the classless category. The Exterior Gateway Protocol (EGP), the predecessor of the
Border Gateway Protocol (BGP), which currently is considered obsolete, is also a classful protocol.
Table 1-3. Classful and Classless IP Routing Protocols