3.6 TCP/IP terminology Host: Any systems attached to an IP network Interface: A logical representation of a network interface card (OSA) Port: An entrance to or exit from a network. The part of a socket address that identifies a port within a host. Socket: A logical entity used to identify a remote application - socket = <IP address> + a port number Connects networks and routes packets Router: between them Figure 3-7 TCP/IP terminology TCP/IP terminology The following terminology is commonly used to describe a TCP/IP environment: Host Interface In the Internet suite of protocols, this is an end system. The end system can be any workstation; it does not have to be a mainframe. A functional unit that interconnects two computer networks with different network architectures. An interface connects networks or systems of different architectures. A bridge interconnects networks or systems with the same or similar architectures. In TCP/IP, this is a synonym for router . 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. The well-known ports are controlled and assigned by the Internet central authority (IANA) and on most systems can only be used by system processes or by programs executed by privileged users. The reason for Chapter 3. TCP/IP stack Port 63