The trick is knowing which services you need and which can be shut down. To reassure yourself that a service is no longer available, you can try connecting to the service. If the server replies with a banner, you know the service is still running. If you've edited the inetd.conf configuration file to remove a service and have forgotten to restart inetd , this will alert you to the fact that the service is still on. In general, this is easy.
Most services run on what are called well-known ports.
If the ports weren't well known, other systems wouldn't know how to make a connection. If POP3, for example, didn't run on port 110, or if this port assignment wasn't well understood, mail clients would not be able to fetch mail on behalf of users. They simply wouldn't know how to make the needed connections. This file is used to associate service ports with names. The default configuration file on a typical Unix system will support many more services than you probably need.