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IP addresses

To allow communication over heterogeneous networks, each with its own addressing standard, every location in a network needs a globally unique IP address. A computer that is connected to the Internet needs at least one IP address; a node that interconnects two networks needs two.

IP addresses are 32-bit numbers that are commonly represented as a series of four decimal numbers between 0 and 255, separated by a period. An example IP address is 134.148.250.28. Some IP addresses have special meanings; for example, the IP addresses 127.0.0.0 and 127.0.0.1 are reserved for loopback testing on a host. If a connection is to be made from a client to server, both running on the same machine, the address 127.0.0.1 can be used. This address loops back to 127.0.0.0, the localhost. The address 0.0.0.0 is used by IP to identify the default route out of a node.

A system's network file contains the links between network devices and IP addresses. The IP network information can usually be found in the file /etc/networks on a Linux system.

Ports

When a virtual connection is set up between two communicating systems, each end is tied to a port. The port is an identifier used by the TCP software rather than an actual physical device, and it allows multiple network connections to be made on one machine by different applications.

When a message is received by the TCP software running on a host computer, the data is sent to the correct application based on the port number. By convention, a well-known port is normally used by a server providing a well-known service. A list of well-known ports for various applications is maintained by Internet Assigned Number Authority (IANA) and can be found at http://www.isi.edu/in-notes/iana/assignments/port-numbers. For example, the File Transfer Protocol (FTP) uses port 21, and a web server uses port 80.

Systems with TCP/IP software installed have a services file that lists the ports used on that machine. This file is often preconfigured for well-known applications and is maintained by the system administrator to reflect the actual port usage on the machine. This file is usually /etc/services on a Linux system.

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