Like a ground-floor window, every network service is a potential portal for burglars or crackers to crawl through, so you should at least know where your ground floor windows are located, even if you only have a screen on them with no lock.
Network services attach themselves to virtual entities called ports.
They are conceptually similar to the serial and printer ports in the back of your machine because they allow data to pass through, but these ports reside in the networking software. There are 65,535 network ports. Ports 1 through 1024 are called privileged ports because only processes running as the superuser are allowed to connect to them. Another distinguishing characteristic of these ports is each is assigned to a specific protocol, such as ftp or telnet. When a network request comes in on a port, inetd will decide which service corresponds to that port and it will start a service to take care of the request.