Classful routing/networking is no valid into today’s CIDR (Classes Inter-Domain Routing) networks; however, it seems that people are still over using the classful terminology almost interchangeably. The concept of classes in network is obsolete and should no longer be used when referencing network subnets because classful routing has defined network ranges that make them classful and CIDR does not.
You hear all too often: I have two “class C’s” or some other form of this. This is plain wrong because in regards to classful routing: 10.5.0.0/24 is not a single class “C” because in regards to classful routing 10.5.0.0 is a “/8”: 10.5.0.0/8. Therefore, the correct terminology that should be used is: I have two /24 subnets in use. I hear this a long time ago when referencing a /23 network and my co-worker at the time at Portbridge Internet kept reusing the term “Two Class C’s” and this made no sense to me because I regarded that network as a /23 subnet.
The point here is that if a ISP hands you a /23 subnet it is possible to break that subnet down into smaller networks and not necessarily just two subnets based upon “Class C’s”. For instance, I could take that /23 at my router and then further break it down into smaller networks that would suit my organization. I can have multiple /25’s, 26’s etc. The point is you need to refrain from using classful terminology because it doesn’t make you look very intelligent in a CIDR world.