Identifying subnet bits of network addresses with same CIDR [closed]

Solution 1:

Classful networking was obsoleted in 1993 by RFC 1518/1519. Please leave network classes in the history drawer where they belong, they are irrelevant.

They all have CIDR of 30.

There is no such a thing. You likely mean prefix length.

So how do I identify the number of subnet bits from looking at their subnet masks or CIDR?

The number of network bits is the number indicated by e.g. /30. Only when you subnet a network with m bits to n bits*, the number of subnet bits is n - m: the number of host bits you repurpose as network bits.

You can subnet each /30 network in two /31 subnets, for point-to-point links only.