At each host, many applications (processes) may be using TCP/UDP at the same time. Each application is uniquely identified by a triple
Fig. 13.5 Applications using TCP
Application = (HostIP, Protocol, PortNumber)
where Protocol is either TCP or UDP, PortNumber is a unique unsigned short integer assigned to the application. In order to use UDP or TCP, an application (process) must choose or obtain a PortNumber first. The first 1024 port numbers are reserved. Other port numbers are available for general use. An application may either choose an available port number or let the OS kernel assign a port number. Figure 13.5 shows some of the applications that use TCP in the transport layer and their default port numbers.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.