What is used to avoid assigning entire addresses of a particular class to networks that didnâ’t have that many hosts?

CIDR (Common Internet Domain Routing). Originally, there were Class A, Class B, Class C and Class D networks. Class A nets had an 8 bit netmask, so they could have 17+ million nodes (whatever 2^24 is). Class B networks had a 16 bit netmask and a  capacity of 65,536 nodes. Class C nets 24 bit netmask, 256 nodes. Class D was usually (I think, lot of cobwebs) 26 bit, 64 node.

When internet numbers started to run low, the protocols were tweaked to accept variable length netmasks, CIDR.