15.6 TRANSPORT PROTOCOLS

The Internet Protocol (IP) delivers packets between computers connected to an internetwork. However, a modern computer often runs many network applications simultaneously, including web browsers, email clients, video conferencing software, file transfer applications, and streaming services. A mechanism is therefore required to ensure that incoming data is delivered to the correct application. This function is performed by the Transport Layer.

The Transport Layer provides end-to-end communication between applications running on different computers. In addition to directing information to the correct application, transport protocols may also provide reliability, sequencing, flow control, and error recovery.

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.