TCP and UDP identify applications using 16-bit port numbers.
Servers are normally known by their well-known port number.Those services that can be provided by any implementation of TCP/IP have well-known port numbers between 1 and 1023.
A client usually doesn't care what port number it uses on its end. All it needs to be certain of is that whatever port number it uses be unique on its host. Client port numbers are called ephemeral ports (i.e., short lived). This is because a client typically exists only as long as the user running the client needs its service, while servers typically run as long as the host is up.
Most TCP/IP implementations allocate ephemeral port numbers between 1024 and 5000. The port numbers above 5000 are intended for other servers (those that aren't well known across the Internet).
The well-known port numbers are contained in the file /etc/services on most Unix systems.