|Interesting ways of looking at it. You have to select a class that will support the number of nodes you need and to allow for growth. Call me silly but I always use Class A for for the private networks and Class B for DMZs. No logic behind this just that there is no costs to using Class A in a private network. |
When it comes to WANs it is a different story entirely. You need to balance the number of networks you have with the number of host on that network. A lot of times you are given a small CIDR block to work with when building your WAN so it becomes very important. Also, if you are going to use Multicasting then you might need a different class entirely.
If I was building a new network from scratch I might just chose to go with IP v6 because you don't have to worry about classes and multicasting.
I am not sure if you are building a LAN or a WAN.