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Mapping Port Names to Numbers

This is a comment line in /etc/services, because it starts with a hash (#) sign. All text after the hash, to the end of the line, is ignored.

Blank lines are ignored in /etc/services. 1) ftp protocol, 2) instant messenger protocol

The first field in /etc/services is the official service name.

The second field in /etc/services is the port and protocol, separated by a forward slash (/)

This is an alias. The words in the third field and beyond are aliases to the official service name in field one.

This is a list of Internet socket port numbers used by protocols of the Transport Layer of the Internet Protocol Suite for the establishment of host-to-host connectivity.
Originally, port numbers were used by the (NCP) Network Control Program in the ARPANET for which two ports were required for half duplex transmission. Later, the (TCP) Transmission Control Protocol and the User Datagram Protocol (UDP) needed only one port for full duplex, bidirectional traffic.
The even numbered ports were not used, and this resulted in some even numbers in the well-known port number range being unassigned. The (SCTP) Stream Control Transmission Protocol and the (DCCP) Datagram Congestion Control Protocol also use port numbers. They usually use port numbers that match the services of the corresponding TCP or UDP implementation, if they exist.
The (IANA) Internet Assigned Numbers Authority is responsible for maintaining the official assignments of port numbers for specific uses. Nevertheless, many unofficial uses of both well-known and registered port numbers occur in practice.