Port (computing)

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This article is about the computer outlet. For software porting, please see Porting.

Image:Ibook-g3900-ports-lg.jpg In computing, a port is a connection through which data is sent and received.

1 Hardware port
2 Network port
3 I/O or machine port mechanism - port-mapped I/O
4 See also
5 External links

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Hardware port

A hardware port is an electrically wired outlet on a piece of equipment into which a plug or cable connects. These hardware ports have different physical shapes such as male, female, round, rectangular, square, oblong, etc. There is some standardization to physical properties and function. For instance, most computers have a keyboard port (currently round, or telephone type), into which the keyboard is connected.

Hardware ports can almost always be divided into two groups:

Serial ports send and receive one bit at a time via a single wire pair (Ground and +/-).
Parallel ports send multiple bits at the same time over several sets of wires.

The system bus that holds circuit cards for a computer "Mother Board (MO)" is a form of parallel port. Note that light fiber, microwave, and other technologies (i.e. quantum) may have special cases.

Hardware port trunking (HPT) is a technology that allows multiple hardware ports to be aggregated into a single group, effectively creating a single connection with a higher bandwidth. This technology also provides a higher degree of fault tolerance. Compare this to Software Port Trunking (SPT) where two agents (websites, channels, etc.) are bonded into one with the same effectiveness, i.e. ISDN B1 (64K) plus B2 (64K) equals data throughput of 128K.

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Network port

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A network port is a special number, ranging from 0-65535, recognized by the TCP and UDP protocols. These protocols use the ports to map incoming data to a particular process running on a computer.

One might describe the idea using the following analogy. Imagine computers having thousands of numbered hardware ports, each with wires sticking out of them. In this virtual scenario, applications could communicate as follows: Application A listens for network traffic arriving on port 90 (for example.) Application B knows that Application A is always listening on port 90, so it sends some data down the corresponding wire. The number of concurrent listening applications is limited by the number of wires there are sticking out of the computer.

This analogy is only for explanation, in reality, however, there may only be one hardware port (the network socket). By including the desired port number in the transmission of data it is possible to target specific applications on the recieving end and as long as they are listening to the correct port number they will receive the correct data, even though all data is sent through the same physical port. In this way we can create the illusion that there are many thousands of available ports.

Whether or not network ports are used depends on the transport layer. Both UDP and TCP use ports.

A port may send/receive data one direction at a time (Half Duplex) or simultaneously in both directions (Full duplex). Software network ports may also connect internal programs on a single computer system.

In TCP and UDP the combination of a port and a network address (IP-number) is called a socket: e.g. the TCP and UDP port numbers.

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I/O or machine port mechanism - port-mapped I/O

For Input or Output (I/O) operations nearly all processor families use similar assembly instructions for both memory access and hardware I/O (see memory-mapped I/O for details). However, Intel microprocessors have assembly instructions (IN and OUT) that are used specifically for hardware I/O. These instructions figure out which hardware device to communicate with using the concept of an I/O port or machine port. These ports are numbered based on which hardware device they refer to.

Intel microprocessors generally allow one octet (8-bit byte or word) to be sent or received during each instruction. The hardware device decides how to interpret data sent to it and what data to send to the processor. For example, a common use is to ask a hardware device which byte (in a data transfer) it will be sending next.

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