Your default class addresses are Class A 0-127, Class B - 128-191 Class C - 192-223 for the 1st octet values

Classful ip addresses are ip addresses that follow this standard subnet ranges for class A, B, C so a classful router protocol like ripv1 will always assume that the address 172.16.1.2 has a subnet mask of 255.255.0.0 even if you want it to have a subnet of 255.255.255.0 so on a classful router protocol 172.16.1.2 will always have the range 172.16.0.0 - 172.16.255.255 (because the value 172 in the 1st octet falls in the Class B range of 128-191 and class B addresses have the subnet mask set to 255.255.0.0)

Classless ip addresses mean that the address range is determined by the subnet mask and hence the same address 172.16.1.2 255.255.255.0 will now be looked at as having its range as 172.16.1.0 - 255 because 255.255.255.0 corresponds to that range. P.S. Rip was updated to support this (ripv2)