Linux Firewalls, Third Edition

Service Ports: The Door to the Programs on Your System

Network-based services are programs running on a machine that other computers on the network can access. The service ports identify the programs and individual sessions or connections taking place. Service ports are the numeric names for the different network-based services. They are also used as numeric identifiers for the endpoints of a particular connection between two programs. Service port numbers range from 0 to 65535.

Server programs (that is, daemons) listen for incoming connections on a service port assigned to them. By historical convention, major network services are assigned well-known, or famous, port numbers in the lower range from 1 to 1023. These port number–to–service mappings ...