The bytes of an IP address are classified into two parts: the network part and the host part. For example, in a typical IP address, such as 129.144.50.56, the first two fields are the network identifier, and the last two fields are the host (computer) identifier.
Depending on the number of hosts you need to support on your network, the network and host parts of your IP addresses may be smaller or larger -- for example, the network part of the address may be three fields, and the host part one field. This in turn determines the class of network you should use.
Currently there are three classes of TCP/IP networks: class A, class B, and class C, with class A networks providing support for the largest number of hosts. Each class uses the 32-bit IP address space differently, providing more or fewer bits for the network part of the address.
Network numbers and classes are assigned by the InterNIC addressing authority; you cannot simply give yourself a network number. Click Help->Help Topics in the AdminSuite main window to find out more about network classes and network addresses.