Nothing actually uses classful networking anymore. Any situation where classful network concepts are implemented is necessarily limiting the capabilities of the network. As such it’s completely useless to bother spending time learning it.

There’s nothing inherently important to classful networking you learn that’s necessary for VLSM. They amount to common convention based on subnet size, and even then nearly nobody actually uses A or B sized subnets except as summary routes, which again, is not inherent to classful networking.

Classful networking has been obsolete for thirty years for good reason, you gain nothing from restricting yourself in that way.

Classful networking is well past dead, that’s kinda pointless. Learn VLSM and general subnetting basics instead.

The whole point of 2.5 and 5 is to support higher speeds over existing lower-rated cabling so I don’t see the point of multigig fiber connections when 10g fiber runs on the same cables that 1g fiber does

They make make it but it seems silly.