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TCP IP Ports Sockets

Ports and sockets

The concepts of the port and socket, which are needed to determine which local process at a given host actually communicates with which process, at which remote host, using which protocol. If this sounds confusing, consider the following points:

An application process is assigned a process identifier number (process ID), which is likely to be different each time that process is started.

Process IDs differ between operating system platforms, thus they are not uniform.

A server process can have multiple connections to multiple clients at a time, thus simple connection identifiers are not unique. The concept of ports and sockets provides a way to uniformly and uniquely identify connections and the programs and hosts that are engaged in them, irrespective of specific process IDs.

The concept of ports and sockets provides a way to uniformly and uniquely identify connections and the programs and hosts that are engaged in them, irrespective of specific process IDs.

Ports

Each process that wants to communicate with another process identifies itself to the TCP/IP protocol suite by one or more ports. A port is a 16-bit number used by the host-to-host protocol to identify to which higher-level protocol or application program (process) it must deliver incoming messages. There are two types of ports:

Well-known: Well-known ports belong to standard servers, for example, Telnet uses port 23. Well-known port numbers range between 1 and 1023 (prior to 1992, the range between 256 and 1023 was used for UNIX-specific servers). Well-known port numbers are typically odd, because early systems using the port concept required an odd/even pair of ports for duplex operations. Most servers require only a single port. Exceptions are the BOOTP server, which uses two: 67 and 68 and the FTP server, which uses two: 20 and 21. The well-known ports are controlled and assigned by the Internet Assigned Number Authority (IANA) and on most systems can only be used by system processes or by programs executed by privileged users. Well-known ports allow clients to find servers without configuration information. The well-known port numbers are defined in STD 2 – Assigned Internet Numbers.