Classful protocols can’t use VLSM (i.e., RIPv1 and IGRP, which are no longer in the CCNA syllabus). This is because they don’t recognise anything other than default network masks:
Router#debug ip rip
RIP protocol debugging is on
01:26:59: RIP: sending v1 update to 255.255.255.255 via Loopback0
192.168.1.1
Classless protocols use VLSM (i.e., RIPv2 and EIGRP):
Router#debug ip rip
RIP protocol debugging is on
01:29:15: RIP: received v2 update from 172.16.1.2 on Serial0
01:29 . . .
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