As soon as multiple UNIX systems in a network want to access common resources, it becomes important that all user and group identities are the same for all machines in that network. The network should be transparent to users: whatever machines they use, they always find themselves in exactly the same environment. This can be done by means of NIS and NFS services. NFS distributes file systems over a network and is discussed in Section 28.0, Sharing File Systems with NFS.
NIS (Network Information Service) can be described as a database-like
service that provides access to the contents of
/etc/passwd, /etc/shadow, and
/etc/group across networks. NIS can also be used for
other purposes (making the contents of files like
/etc/hosts or /etc/services
available, for example), but this is beyond the scope of this
introduction. People often refer to NIS as YP,
because it works like the network's yellow pages.