NIS+ is not only an upgrade to NIS; it is designed to replace NIS. This becomes evident when you examine its domain structure. NIS domains are flat and lack the ability to have a hierarchy. NIS+ domains may be flat, but you can also construct hierarchical NIS+ domains. Such hierarchies consist of a root domain with an infinite number of subdomains under them.
The NIS domain structure addressed the administration requirements of client-server computing networks prevalent in the 1980s, in other words, client-server networks with a few hundred clients and a few multipurpose servers.
NIS+ is designed to support networks with 100 to 10,000 clients supported by 10 to 100 specialized servers located in sites throughout the world, connected to several "untrusted" public networks. The size and complexity of these networks requires new, autonomous administration practices. The NIS+ domain structure was designed to address these requirements. It consists of hierarchical domains similar to those of DNS, as shown in the following diagram:
Hierarchical domains allow NIS+ to be used in a range of networks, from small to very large. They also allow the NIS+ service to adapt to the growth of an organization. The NIS+ domain structure is thoroughly described in Solaris Naming Administration Guide.