At level 3, the addressing (this means the IP address) uniquely identifies a station located anywhere on the network. Once the station is found, it must be able to determine the application that will process the data. The port number does this. The servers, whatever the station, have fixed port numbers, which are well known. The application for port numbering is generally distributed to every system. The IANA1 website has a list of port numbers attributed to well-known applications (see Table 8.1).
The numbers from 0 to 1,023 are reserved. The servers working over a Unix station have to be initiated at the same time as the station2 or initialization has to be carried out by the superusers. If this was not the case, any programmer could create a false telnet server which a remote user would connect to in confidence and would give their password.
The numbers between 1,024 and 49,151 are used by IANA registered applications3.
If the server fixes the port number. On the client side, the number is generally dynamically chosen from among the free numbers. The range between 49,152 and 65,535 ...