Private TCP/IP addresses

What should I use for my TCP/IP addresses?

Network Solutions set aside several sets of addresses for use internally. These addresses are legal IP addresses, but not legal "Internet" IP addresses and are not used on the Internet because they were set aside. If you attempt to ping or trace route to one of these IP address on the Internet you will either receive no answer, or a special reply which Network Solutions recently implemented, pointing you to the RFC that explains the IP addresses for private networks, RFC1597.

The addresses are as follows, in an excerpt from RFC1597 (Request For Comments)

Begin Excerpt

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks:

10.0.0.0 - 10.255.255.255

172.16.0.0 - 172.31.255.255

192.168.0.0 - 192.168.255.255

We will refer to the first block as "24-bit block", the second as "20-bit block, and to the third as "16-bit" block. Note that the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 255 contiguous class C network numbers. An enterprise that decides to use IP addresses out of the address space defined in this document can do so without any coordination with IANA or an Internet registry. The address space can thus be used by many enterprises. 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.

End Excerpt

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 could start with a class B subnet that you subnet down to a class C, then you can expand it later by only changing the subnet mask on your current installations.

Please note: If one computer is on one subnet and another computer is on another subnet you will need a router between them (their network segments) in order for one computer to reach the other. Example: My subnet mask is 255.255.255.0 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. If I have an address of 172.30.1.1 and the other computer has an address of 172.30.2.1, I will see it on my network because my net mask says that the first 2 octets are my network number and the last two are my node number. If the other computer has an address of 172.31.1.1 then I will need a router to see it.