NIS (Network Information System) in the current name for what was once
known as yp (Yellow Pages). The purpose of NIS is to allow many
machines on a network to share configuration information, including
password data. NIS is not designed to promote system security. If
your system uses NIS you will have a very short /etc/passwd file that
includes a line that looks like this:

+::0:0:::i

To view the real password file use this command `ypcat passwd`

NIS and NIS+ are being replaced by LDAP (Lightweight Directory Access Protocol).