Addresses within this private address space will only be unique within the enterprise. As before, any enterprise that needs globally unique address space is required to obtain such addresses from an Internet registry. An enterprise that requests IP addresses for its external connectivity will never be assigned addresses from the blocks defined above. In order to use private address space, an enterprise needs to determine which hosts do not need to have network layer connectivity outside the enterprise in the foreseeable future. Such hosts will be called private hosts, and will use the private address space defined above.
Private hosts can communicate with all other hosts inside the enterprise, both public and private.
However, they cannot have IP connectivity to any external host. While not having external network layer connectivity private hosts can still have access to external services via application layer relays. So from this document I would suggest that you start out with your choice of the class C network addresses and if you see a need for more subnets you can add another class C address or move to a class B address. Then again, you coul start with a class B subnet that your subnet down to a class C, then you can expand it later by only changing the subnet mask on your current installations. If computer A has an IP address of 198.22.1.1 and the computer I want to reach has an address of 198.22.2.1, I will need a router to see the computer on the network because my subnet mask says that the first 3 octets are my network number and the last is my node number.