Final answer:
The classful routing protocol that does not support VLSM or subnetting is RIPv1. It is an older protocol in comparison to RIPv2, EIGRP, and OSPF, which are classless and support VLSM.
Step-by-step explanation:
The routing protocol described as a classful protocol that does not support Variable Length Subnet Masking (VLSM) or CIDR (Classless Inter-Domain Routing) is RIPv1 (Routing Information Protocol version 1). This protocol is an older routing protocol that does not send subnet mask information in its routing updates. Hence, it cannot support the more efficient VLSM that allows different subnet masks to be used within the same network. This is contrary to classless routing protocols, such as RIPv2, EIGRP (Enhanced Interior Gateway Routing Protocol), and OSPF (Open Shortest Path First), which do support VLSM and are thus able to more efficiently utilize IP address space.