15.6.1 Port Numbers
Each network application is identified by a port number.
When an application sends data, the transport protocol includes both a source port and a destination port within the transport header. These port numbers allow multiple applications to communicate simultaneously over the same network connection without interfering with one another.
For example, a web browser, an email program, and a video-conferencing application can all exchange information with different remote servers at the same time because each communication session uses a different combination of port numbers.
Port numbers are 16-bit values, allowing 65,536 possible ports (0–65535). Well-known applications are commonly assigned standardized port numbers. For example: HTTP (Web) Port 80; HTTPS (Secure Web) Port443; SMTP (Email) Port 25; and DNS Port 53. The combination of an IP address and a port number uniquely identifies a communication endpoint within the Internet.
Back to reading