Mac OS X: What Is a Port?

What is a "port?"

A port is a location for passing data in and out of your computer.

Ports take both physical and non-physical forms, and they fall into three major categories related to Mac OS X and the computers that use them:

1. device ports
2. network ports
3. IP ports

The first two kinds are physical ports used to connect to your devices and to your network, seen here on an iMac:

Device ports

These ports allow you to connect to video, storage, and input-output devices, such as monitors, disk drives, mice, keyboards, and printers. Figures 1 and 2 illustrate the ports found on two different models of iMac computer. Note that port selection does vary on Mac OS X-compatible computers. Ports that are for devices include:

• Universal Serial Bus (USB)
• FireWire
• Mini-VGA Video
• Sound Input
• Sound Output (or Headphone)
• Apple Speaker Sound Output
• Apple Display Connector (not pictured)

Important synonyms: In documentation at Apple or elsewhere, you may also see some of these ports referred to as "jacks" or "sockets." These terms often appear in association with more traditional terms for the type of wire or cable connector that is being used, such as a "headphone jack."

Network ports

The Ethernet and modem ports shown above are examples of ports used to connect to a network. An AirPort card is also a network port. The System Preferences application in Mac OS X refers to these as "network ports," as seen here:

Important synonym: Sometimes Mac OS X also refers to these as "network interfaces," such as in the Network Utility application. "Network interface" is a common term that means the same thing.

IP ports

You may not be aware of IP ports very often, but you probably use them every day. Servers often deliver more than one type of service, so using the Internet address (URL) of a server is not enough -- you must also tell the server what you want. These requests are made by "port" number. Web service (HTTP) is commonly delivered on port 80, for example. Web browsers are programmed to assume that you want port 80 when you type a URL, such as "www.apple.com". That is why you do not need to be aware of which port you are using for most Web browsing.

Sometimes you need to type a port number when connecting to a service. Whether you must depends on what client software you are using, the service, and how the server is set up. You could type a port number when opening a QuickTime Streaming Server stream with iTunes, like this:

IP stands for "Internet protocol," which can be subdivided into port types such as TCP and UDP. For more on these ports, see "Well-Known" TCP and UDP ports used by Apple software products.