What are ports in computing and how do they work?

What is a computer network port?

A port in computer networking is how a computer can use a single physical network connection to handle many incoming and outgoing requests by assigning a port number to each. The numbers go from 0 to 65535, which is a 16-bit number.

Some of these port numbers are specifically defined and always associated with a specific type of service — for example, File Transfer Protocol (FTP) is always port number 21 and Hypertext Transfer Protocol web traffic is always port 80. These are called well-known ports and go from 0 to 1023.

The numbers from 1024 to 49151 are called registered ports and can be registered with the Internet Assigned Numbers Authority for a specific use. The numbers 49152 to 65535 are unassigned, can be used by any type of service and are called dynamic ports, private ports or ephemeral ports.

As an illustration, think of a port number as a telephone extension in a business telephone system. The company telephone number is like the computer Internet Protocol (IP) address or Uniform Resource Locator (URL). It connects users with the correct company but does not define who they want to talk to in the company. A telephone extension will connect them with the right person in an organization, however, and the port will connect them to the right service on a computer. Dialing extension 0 to speak to an operator is extremely common in all phone systems and is like the well-known ports that always define specific services.

Port numbers identify specific applications or services on a computer system, just like an IP address identifies a machine in an IP network and ascertains the destination of a data packet.

A port is always associated with a protocol. Generally, this is with Transmission Control Protocol or User Datagram Protocol for communication, but Internet Control Message Protocol messages use specific ports as well. The port is specified by having the URL or IP address followed by a colon then the port number — as examples, 10.0.0.1:80 or www.techtarget.com:443. With all internet communication, there is always an associated port, but it may not be shown to the user as it is often implied by the type of communication.

A computer can manage many simultaneous connections on a single inbound port. This is because the local IP address, local port, remote IP address and remote port specify each connection. A listening port is when the computer is actively waiting for inbound requests on that port number, allowing those connections. Port forwarding is when communication to one address on a specific port is then sent, or forwarded, to another computer for processing.