The master database also contains entries for all network services that require Kerberos authentication. Suppose that your site has a machine, laughter.mit.edu, that requires Kerberos authentication from anyone who wants to rlogin to it. The host's Kerberos realm is ATHENA.MIT.EDU.
This service must be registered in the Kerberos database, using the proper service name, which in this case is the principal:
The / character separates the Kerberos primary (in this case, host) from the instance (in this case, laughter.mit.edu); the @ character separates the realm name (in this case, ATHENA.MIT.EDU) from the rest of the principal. The primary, host, denotes the name or type of the service that is being offered: generic host-level access to the machine. The instance, laughter.mit.edu, names the specific machine that is offering this service. There will generally be many different machines, each offering one particular type of service, and the instance serves to give each one of these servers a different Kerberos principal.