1. In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint used by Transport Layer protocols of the Internet Protocol Suite, such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
2. A field in the TCP & UDP headers that specifies which applications are sending & receiving the messages. The port field is the layer interface that enables TCP & UDP to transfer messages to any application.
3. A port number is a number assigned to a particular network service on a host. For example, SMTP usually uses port 25, while HTTP is usually port 80.
4. A field in a TCP or UDP header that identifies the application that either sent (source port) or should receive (destination port) the data inside the data segment.
5. A port number identifies both a computer and also a "channel" within that computer where network communication will take place.
6. In computer networking, a port number is part of the addressing information used to identify the senders and receivers of messages. Port numbers are most commonly used with TCP/IP connections. Home network routers and computer software work with ports and sometimes allow you to configure port number settings. These port numbers allow different applications on the same computer to share network resources simultaneously.
7. Port numbers are associated with network addresses. For example, in TCP/IP networking, both TCP and UDP utilize their own set of ports that work together with IP addresses.
8. Port numbers work like telephone extensions. Just as a business telephone switchboard can use a main phone number and assign each employee an extension number (like x100, x101, etc.), so a computer has a main address and a set of port numbers to handle incoming and outgoing connections.
9. In both TCP and UDP, port numbers start at 0 and go up to 65535. Numbers in the lower ranges are dedicated to common Internet protocols (like 21 for FTP, 80 for HTTP, etc.).
A well-known range of port numbers is reserved by convention to identify specific service types on a host computers. In the client-server model of application architecture this is used to provide a multiplexing service on each port number that network clients connect to for service initiation, after which communication is reestablished on other connection-specific port numbers.
Examples
An example for the use of ports is the Internet mail system. A server used for sending and receiving email generally needs two services. The first service is used to transport email to and from other servers. This is accomplished with the Simple Mail Transfer Protocol (SMTP). The SMTP service application usually listens on TCP port 25 for incoming requests. The second service is the Post Office Protocol (POP) which is used by e-mail client applications on user's personal computers to fetch email messages from the server. The POP service listens on TCP port number 110. Both services may be running on the same host computer, in which case the port number distinguishes the service that was requested by a remote computer, be it a user's computer or another mail server.
While the listening port number of a server is well defined (IANA calls these the well known ports), the client's port number is often chosen from the dynamic port range (see below). In some applications, the client and the server each use specific port numbers assigned by the IANA. A good example of this is DHCP in which the client always uses UDP port 68 and the server always uses UDP port 67.
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